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Journal of the History of the Neurosciences: Basic and Clinical Perspectives Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/njhn20
Letter to the Editor: Reexamining Penfield’s Homunculus Zina Ward
a
a
Emmanuel College, Cambridge, United Kingdom Published online: 16 Apr 2014.
To cite this article: Zina Ward (2014) Letter to the Editor: Reexamining Penfield’s Homunculus, Journal of the History of the Neurosciences: Basic and Clinical Perspectives, 23:2, 198-203, DOI: 10.1080/0964704X.2013.868972 To link to this article: http://dx.doi.org/10.1080/0964704X.2013.868972
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http://www.tandfonline.com/page/termsand-conditions
Journal of the History of the Neurosciences, 23:198–203, 2014 Copyright © Taylor & Francis Group, LLC ISSN: 0964-704X print / 1744-5213 online DOI: 10.1080/0964704X.2013.868972
Letter to the Editor: Reexamining Penfield’s Homunculus ZINA WARD
Downloaded by [University of Tasmania] at 09:46 02 September 2014
Emmanuel College, Cambridge, United Kingdom Snyder and Whitaker’s (2013) discussion of Penfield’s homunculus in a recent issue of this journal was stimulating but I believe mistaken in several ways. This letter clarifies the diagram’s supposed ambiguity, highlights a largely overlooked problem with improper scaling and challenges the authors’ argument for the superiority of the 1950 version of the image. Keywords
Wilder Penfield, homunculus, improper scaling
In their recent article in this journal (2013, Vol. 22, No. 3), Peter J. Snyder and Harry A. Whitaker offer an admiring portrait of Dr. Wilder Penfield but a critical assessment of his “cerebral cartography,” adding their voices to a growing chorus of skepticism about the value of Penfield’s homunculus (Schott, 1993; Pogliano, 2012). Although Snyder and Whitaker helpfully discuss several of the homunculus’ genuine limitations, I believe their remarks contain several exaggerations as well as one significant omission. A common accusation against the iconic image, echoed by Snyder and Whitaker, is that it is not clear what exactly the homunculus represents. In discussing the 1937 version, the authors claim that what Penfield and Boldrey (1937) meant by “vertical extent of the Rolandic cortex” is uncertain (Snyder & Whitaker, 2013, p. 283). They argue that all of the homunculi are open to several possible interpretations: “Were the primary design features chosen to represent relative differences in two-dimensional surface area, the lateral extent of each somatotopic correlate, the depth of each zone in three dimensions, or the magnitude of intersubject variability?” (Snyder & Whitaker, 2013, p. 286). This list of possibilities, as well as an earlier one targeted at the 1937 homunculus (Snyder & Whitaker, 2013, p. 283), is overly imaginative. While Penfield and Boldrey could undoubtedly have expressed themselves more clearly, it is apparent from the figures’ legends and surrounding text that what is being illustrated is a characteristic of the cortical surface. It is true, as Snyder and Whitaker point out (2013, p. 283), that Penfield did occasionally stimulate below the surface after cortical ablations. However, parts of the Rolandic cortex were rarely excised (Penfield & Boldrey, 1937, p. 396) and most of the sensorimotor mapping was done in preparation for excision, not after it (Penfield & Rasmussen, 1950, p. 6). It is therefore implausible that the homunculus was meant to convey information about depth or volume. Nor was variation the target. Snyder and Whitaker believe that Penfield and Boldrey’s specification, “in individual cases,” supports this potential interpretation (2013, p. 283). In fact, it does the exact opposite. By indicating that the size of the homunculus’ body parts are to be compared within the same individual, Penfield and Address correspondence to Zina Ward, Emmanuel College, St. Andrew’s Street, Cambridge, CB2 3AP UK. E-mail: [email protected]
198
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Boldrey suggest that the homunculus represents the average subject. This clarification distinguishes the homunculus from an image featured earlier in their paper that does represent interpersonal variation (see Fig. 1), showing as it does the regions “within which responses were obtained for each subdivision of the body” in all patients (Penfield & Boldrey, 1937, p. 430). Any lingering doubts about the kind of information conveyed by the homunculus are dispelled by Penfield and Rasmussen (1950), who state in a legend that “the lengths of the solid bars [underneath the figurine] represent an estimate of the average relative cortical areas from which the corresponding responses were elicited” (p. 24, emphasis added). The homunculus is thus not about depth, variability, or merely lateral extent: It is unequivocally about cortical area. This conclusion necessitates a slight correction to Snyder and Whitaker’s later explanation of the 1950 homunculus, which they claim contains “solid bars to show the relative amount of cortical territory (in the rostral-caudal orientation) devoted to each portion of the body” (2013, p. 284). The aforementioned legend indicates that the rostral-caudal direction is only half of it: Since the quantity represented is an area rather than a length, the perpendicular direction on the cortical surface (the dorsal-ventral orientation) matters as well. Avoiding ambiguity in a scientific illustration, however, requires not only clarity about what is being represented but also about what feature of the drawing is “doing the representing.” In this case, the former has received too much attention while the latter has been
Figure 1. An illustration depicting intersubject variation in Penfield and Boldrey (1937; their Fig. 25). The original caption reads in part, “Summary of sensory and motor findings . . . The lines enclose the areas within which responses were obtained for each subdivision of the body” (p. 430). ©[Osler Library of the History of Medicine]. Reproduced by permission of Osler Library of the History of Medicine, McGill University. Permission to reuse must be obtained from the rightsholder. (Color figure available online.)
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neglected. The homunculus is undoubtedly trying to convey information about relative cortical area, but it is not immediately clear whether cortical area corresponds to the length, width, area, or volume of the drawn body parts. The only commentator to have remarked on this problem, to my knowledge, is G. D. Schott (1993), who says in passing, “whilst the [1937] homunculus appears to relate to length rather than width, the parts illustrated are in fact enlarged in both directions” (p. 329). Schott’s suspicion that Penfield’s data are reflected in the length of the homunculus receives support from several sources. First, the addition of underlying lines in 1950 and Penfield’s comment that these lines are “more accurate” than the overlaid body parts suggest that length had always been the relevant feature (Penfield & Rasmussen, 1950, p. 44). Furthermore, a draft of the homunculus drawn by Penfield himself (Fig. 2), unpublished and held at the Penfield Archive in Montreal, suggests that Penfield sketched the human figure after he had marked lengths for each body part along a single straight line. Presumably these markings were placed to reflect the average cortical areas observed in Penfield’s data. Strictly speaking, then, the length of the homunculus’ body parts seems to be the information-containing feature of the image. However, if this understanding of Penfield’s homunculus is correct, it raises a major problem for the diagram that is well known in the field of data visualization but rarely discussed in the context of the homunculus: improper scaling. A figure commits the sin of improper scaling when it depicts two- or three-dimensional objects that communicate information with one dimension but vary arbitrarily in another.1 Usually, this happens when the height of an object varies in proportion to the data while the width and/or depth are adjusted to maintain the object’s proportions. Such adjustments can be misleading because the ratio of the objects’ heights may be different from the ratio of their areas or volumes. Readers’ visual impressions are naturally determined by areas and volumes rather than lengths, so figures that contain improper scaling give a mistaken impression of the data. Penfield’s homunculus is guilty of this. For example, the vertical length of the 1937 homunculus’ foot looks to be about three times the vertical length of its ring finger. The foot’s area, however, is more like 10 times the area of the finger. The foot’s relative importance is therefore overstated.2 Likewise with the ubiquitous 1950 version of the image, the ratios of the underlying lines are different from the ratios of the body parts’ areas. That the homunculus’ improper scaling has been overlooked by several generations of critics is surprising. It seems that, if we are to take the problem seriously, those who reprint the familiar 1950 homunculus must redraw the human figurine so that the areas of its parts are directly proportional to the lengths of the corresponding lines. My final comment relates to Snyder and Whitaker’s relative assessments of the different versions of the homunculus. Like most other commentators (Schott, 1993; Pogliano, 2012), they convey a clear preference for the 1950 homunculi, calling them “Penfield’s first, and arguably his last, real visual maps of human cortical functional anatomy, to be created with the intent of displaying this information with scientific accuracy as a primary goal” (Synder & Whitaker, 2013, p. 284). Expressing a general consensus, they argue that, after 1950, Penfield was seduced by artistry and simplicity, abandoning the “appropriate inductive approach” in the construction of future homunculi (p. 290). They accuse the 1954 sensorimotor homunculus of being “crudely drawn,” only loosely based on data, and 1 The classic book How to Lie with Statistics gives a very basic overview of this common error (Huff, 1954, p. 66). 2 I see the figures as two rather than (implicitly) three dimensional, but their perceived dimensionality is certainly debatable. If we adopt the three-dimensional perspective, the problem is even worse: The ratio of the inferred volumes of foot and finger is even greater than the ratio of their areas.
201
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Figure 2. Sketch of the homunculus by Penfield, probably c. 1951. ©Osler Library of the History of Medicine. Reproduced by permission of Osler Library of the History of Medicine, McGill University. Permission to reuse must be obtained from the rightsholder.
Downloaded by [University of Tasmania] at 09:46 02 September 2014
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useless even as a mnemonic, asking “of what in memory these could [sic] be possibly be aiding, if all or most of the details of the drawings were not based on established functional anatomical correlates?” (p. 285). While I will not defend the 1954 thalamic homunculus, I do think that this assessment of the 1954 sensorimotor homunculus is far too harsh. The fact that it is indeed based on an analysis of Penfield’s data can be seen in its differences from the 1950 version, which reveal conscientious updating. First, there is the obvious addition of the secondary and supplementary areas: No longer was Penfield willing to ignore the important role played by extra-Rolandic cortex in somatic sensation and movement. Greater attention was also paid to lateralization in this homunculus. In the Rolandic cortex, only the right half of the body is shown since most responses elicited there were contralateral. In contrast, ipsilateral and bilateral responses were often observed in the secondary sensory and supplementary motor areas, which is why the figurines overlaid on these regions are whole (Penfield & Jasper, 1954, pp. 87, 103). The right half of both figurines is enlarged, however, reflecting the fact that contralateral responses were nevertheless still more common than ipsilateral ones. Other details in the image that reveal its careful, empirically constrained construction, such as the second sensory figurine’s exaggerated fingertips, are pointed out in the figure legend (Penfield & Jasper, 1954, p. 105). Snyder and Whitaker’s assertion that the 1954 sensorimotor homunculus is not even useful as a memory aid therefore seems extreme. It is based on Penfield’s stimulation data. It is visually engaging. It may be imprecise, with locations and sizes only approximately depicted, but precision is not required of a memory device. Indeed, one can hardly be expected to remember the exact lengths of the underlying lines in the 1950 homunculus; such details are superfluous when it comes to memorability. The 1954 version is therefore fully capable of fulfilling its stated purpose of aiding memory.3 I would like to offer a final suggestion that goes even further: I believe, contrary to the conclusion of Snyder and Whitaker and the prevailing consensus, that the 1954 homunculus is actually better than the 1950 version. This is because Penfield’s findings must be strongly qualified in several ways. First, his methods were crude and artificial, as many researchers and clinicians have since pointed out (Walshe, 1953; Schott, 1993; Schieber, 2001; Di Noto et al., 2012). Electrical stimulation — especially the strong stimulation used in Penfield’s era — is a poor imitation of genuine physiological processes, so, although it can be useful, one must be careful in drawing conclusions about normal functioning from it. Second, Penfield’s studies were all conducted on epileptics, raising concerns about generalizability. It is quite likely that there had been some degree of cortical reorganization following brain damage in most of Penfield’s patients, leading Snyder and Whitaker (and many others) to doubt the external validity of his results (2013, p. 288). The possibility of reorganization more generally — not just in pathological populations — also supports a cautious approach to claims about cortical organization. Both lesion- and use-dependent plasticity can alter the location and size of sensorimotor representations, so Penfield’s findings for each individual must not be considered final or fixed. Finally, Penfield’s stimulations often elicited responses from more than one body part at a time, as he and his coauthors amply acknowledge (e.g., Penfield & Boldrey, 1937, p. 430; Penfield & Rasmussen, 1950, p. 42). That different body parts did not have entirely distinct cortical representations is therefore an important caveat. These complications suggest that drawing any type of precise conclusion from Penfield’s studies about the location or size of sensorimotor representations would be a 3 In their concluding statement, Snyder and Whitaker seem to backpedal on this point, acknowledging the homunculi’s heuristic value (2013, p. 289).
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mistake. The cortical points that elicit somatic responses when strongly stimulated do not necessarily delineate a fixed region normally involved in the movement or sensation of those body part or parts. A scientific image summarizing Penfield’s findings ought not, then, convey exactitude. To do so would be to mislead the reader and to indulge in a false presumption to precision — in a sense, sacrificing accuracy for precision. The 1950 homunculus can be accused of this: Penfield and Rasmussen claim to have “corrected” the approximation of the figurine with precise underlying lines (1950, p. 214). The 1954 homunculus, in contrast, is cartoonish through-and-through: From the figurines’ expressions, to the scarcity of text, to the zoomed-out perspective, all aspects of the diagram suggest that it is meant to be merely schematic. This is not because, as Snyder and Whitaker suggest, artistic interest was a higher priority for Penfield than scientific accuracy. Rather, Penfield’s data did not warrant detailed, exact claims about cortical organization. The 1954 homunculus appropriately reflects that.
Acknowledgements My thoughts on the homunculus were shaped by time recently spent at the Osler Library in Montreal. Many thanks to the Mary Louise Nickerson Fellowship and those who helped me navigate the Penfield Archive, especially the late Dr. William Feindel.
References Di Noto PM, Newman L, Wall S, Einstein G (2012): The hermunculus: What is known about the representation of the female body in the brain? Cerebral Cortex 23(5): 1005–1013. Huff D (1954): How to Lie with Statistics. London, Norton, W. W. & Company, Inc., 1st edition. Penfield W, Boldrey E (1937): Somatic motor and sensory representation in the cerebral cortex of man as studied by electrical stimulation. Brain 60: 389–443. Penfield W, Jasper H (1954): Epilepsy and the Functional Anatomy of the Human Brain. Boston, Little, Brown and Co. Penfield W, Rasmussen T (1950): The Cerebral Cortex of Man: A Clinical Study of Localization of Function. New York, The Macmillan Company. Pogliano C (2012): Penfield’s homunculus and other grotesque creatures from the Land of If. Nuncius 27(1): 141–162. Schieber MH (2001): Constraints on somatotopic organization in the primary motor cortex. Journal of Neurophysiology 86(5): 2125–2143. Schott GD (1993): Penfield’s homunculus: A note on cerebral cartography. Journal of Neurology, Neurosurgery, and Psychiatry 56(4): 329–333. Snyder PJ, Whitaker HA (2013): Neurologic heuristics and artistic whimsy: The cerebral cartography of Wilder Penfield. Journal of the History of the Neurosciences 22(3): 277–291. Walshe FMR (1953): Some problems of method in neurology. Canadian Medical Association Journal 68(1): 21–29.
Journal of the History of the Neurosciences: Basic and Clinical Perspectives Publication details, including instructions for authors and subscription information: http://www.tandfonline.com/loi/njhn20
Letter to the Editor: Reexamining Penfield’s Homunculus Zina Ward
a
a
Emmanuel College, Cambridge, United Kingdom Published online: 16 Apr 2014.
To cite this article: Zina Ward (2014) Letter to the Editor: Reexamining Penfield’s Homunculus, Journal of the History of the Neurosciences: Basic and Clinical Perspectives, 23:2, 198-203, DOI: 10.1080/0964704X.2013.868972 To link to this article: http://dx.doi.org/10.1080/0964704X.2013.868972
PLEASE SCROLL DOWN FOR ARTICLE Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and should be independently verified with primary sources of information. Taylor and Francis shall not be liable for any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of the Content. This article may be used for research, teaching, and private study purposes. Any substantial or systematic reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http://www.tandfonline.com/page/termsand-conditions
Journal of the History of the Neurosciences, 23:198–203, 2014 Copyright © Taylor & Francis Group, LLC ISSN: 0964-704X print / 1744-5213 online DOI: 10.1080/0964704X.2013.868972
Letter to the Editor: Reexamining Penfield’s Homunculus ZINA WARD
Downloaded by [University of Tasmania] at 09:46 02 September 2014
Emmanuel College, Cambridge, United Kingdom Snyder and Whitaker’s (2013) discussion of Penfield’s homunculus in a recent issue of this journal was stimulating but I believe mistaken in several ways. This letter clarifies the diagram’s supposed ambiguity, highlights a largely overlooked problem with improper scaling and challenges the authors’ argument for the superiority of the 1950 version of the image. Keywords
Wilder Penfield, homunculus, improper scaling
In their recent article in this journal (2013, Vol. 22, No. 3), Peter J. Snyder and Harry A. Whitaker offer an admiring portrait of Dr. Wilder Penfield but a critical assessment of his “cerebral cartography,” adding their voices to a growing chorus of skepticism about the value of Penfield’s homunculus (Schott, 1993; Pogliano, 2012). Although Snyder and Whitaker helpfully discuss several of the homunculus’ genuine limitations, I believe their remarks contain several exaggerations as well as one significant omission. A common accusation against the iconic image, echoed by Snyder and Whitaker, is that it is not clear what exactly the homunculus represents. In discussing the 1937 version, the authors claim that what Penfield and Boldrey (1937) meant by “vertical extent of the Rolandic cortex” is uncertain (Snyder & Whitaker, 2013, p. 283). They argue that all of the homunculi are open to several possible interpretations: “Were the primary design features chosen to represent relative differences in two-dimensional surface area, the lateral extent of each somatotopic correlate, the depth of each zone in three dimensions, or the magnitude of intersubject variability?” (Snyder & Whitaker, 2013, p. 286). This list of possibilities, as well as an earlier one targeted at the 1937 homunculus (Snyder & Whitaker, 2013, p. 283), is overly imaginative. While Penfield and Boldrey could undoubtedly have expressed themselves more clearly, it is apparent from the figures’ legends and surrounding text that what is being illustrated is a characteristic of the cortical surface. It is true, as Snyder and Whitaker point out (2013, p. 283), that Penfield did occasionally stimulate below the surface after cortical ablations. However, parts of the Rolandic cortex were rarely excised (Penfield & Boldrey, 1937, p. 396) and most of the sensorimotor mapping was done in preparation for excision, not after it (Penfield & Rasmussen, 1950, p. 6). It is therefore implausible that the homunculus was meant to convey information about depth or volume. Nor was variation the target. Snyder and Whitaker believe that Penfield and Boldrey’s specification, “in individual cases,” supports this potential interpretation (2013, p. 283). In fact, it does the exact opposite. By indicating that the size of the homunculus’ body parts are to be compared within the same individual, Penfield and Address correspondence to Zina Ward, Emmanuel College, St. Andrew’s Street, Cambridge, CB2 3AP UK. E-mail: [email protected]
198
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Boldrey suggest that the homunculus represents the average subject. This clarification distinguishes the homunculus from an image featured earlier in their paper that does represent interpersonal variation (see Fig. 1), showing as it does the regions “within which responses were obtained for each subdivision of the body” in all patients (Penfield & Boldrey, 1937, p. 430). Any lingering doubts about the kind of information conveyed by the homunculus are dispelled by Penfield and Rasmussen (1950), who state in a legend that “the lengths of the solid bars [underneath the figurine] represent an estimate of the average relative cortical areas from which the corresponding responses were elicited” (p. 24, emphasis added). The homunculus is thus not about depth, variability, or merely lateral extent: It is unequivocally about cortical area. This conclusion necessitates a slight correction to Snyder and Whitaker’s later explanation of the 1950 homunculus, which they claim contains “solid bars to show the relative amount of cortical territory (in the rostral-caudal orientation) devoted to each portion of the body” (2013, p. 284). The aforementioned legend indicates that the rostral-caudal direction is only half of it: Since the quantity represented is an area rather than a length, the perpendicular direction on the cortical surface (the dorsal-ventral orientation) matters as well. Avoiding ambiguity in a scientific illustration, however, requires not only clarity about what is being represented but also about what feature of the drawing is “doing the representing.” In this case, the former has received too much attention while the latter has been
Figure 1. An illustration depicting intersubject variation in Penfield and Boldrey (1937; their Fig. 25). The original caption reads in part, “Summary of sensory and motor findings . . . The lines enclose the areas within which responses were obtained for each subdivision of the body” (p. 430). ©[Osler Library of the History of Medicine]. Reproduced by permission of Osler Library of the History of Medicine, McGill University. Permission to reuse must be obtained from the rightsholder. (Color figure available online.)
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Zina Ward
neglected. The homunculus is undoubtedly trying to convey information about relative cortical area, but it is not immediately clear whether cortical area corresponds to the length, width, area, or volume of the drawn body parts. The only commentator to have remarked on this problem, to my knowledge, is G. D. Schott (1993), who says in passing, “whilst the [1937] homunculus appears to relate to length rather than width, the parts illustrated are in fact enlarged in both directions” (p. 329). Schott’s suspicion that Penfield’s data are reflected in the length of the homunculus receives support from several sources. First, the addition of underlying lines in 1950 and Penfield’s comment that these lines are “more accurate” than the overlaid body parts suggest that length had always been the relevant feature (Penfield & Rasmussen, 1950, p. 44). Furthermore, a draft of the homunculus drawn by Penfield himself (Fig. 2), unpublished and held at the Penfield Archive in Montreal, suggests that Penfield sketched the human figure after he had marked lengths for each body part along a single straight line. Presumably these markings were placed to reflect the average cortical areas observed in Penfield’s data. Strictly speaking, then, the length of the homunculus’ body parts seems to be the information-containing feature of the image. However, if this understanding of Penfield’s homunculus is correct, it raises a major problem for the diagram that is well known in the field of data visualization but rarely discussed in the context of the homunculus: improper scaling. A figure commits the sin of improper scaling when it depicts two- or three-dimensional objects that communicate information with one dimension but vary arbitrarily in another.1 Usually, this happens when the height of an object varies in proportion to the data while the width and/or depth are adjusted to maintain the object’s proportions. Such adjustments can be misleading because the ratio of the objects’ heights may be different from the ratio of their areas or volumes. Readers’ visual impressions are naturally determined by areas and volumes rather than lengths, so figures that contain improper scaling give a mistaken impression of the data. Penfield’s homunculus is guilty of this. For example, the vertical length of the 1937 homunculus’ foot looks to be about three times the vertical length of its ring finger. The foot’s area, however, is more like 10 times the area of the finger. The foot’s relative importance is therefore overstated.2 Likewise with the ubiquitous 1950 version of the image, the ratios of the underlying lines are different from the ratios of the body parts’ areas. That the homunculus’ improper scaling has been overlooked by several generations of critics is surprising. It seems that, if we are to take the problem seriously, those who reprint the familiar 1950 homunculus must redraw the human figurine so that the areas of its parts are directly proportional to the lengths of the corresponding lines. My final comment relates to Snyder and Whitaker’s relative assessments of the different versions of the homunculus. Like most other commentators (Schott, 1993; Pogliano, 2012), they convey a clear preference for the 1950 homunculi, calling them “Penfield’s first, and arguably his last, real visual maps of human cortical functional anatomy, to be created with the intent of displaying this information with scientific accuracy as a primary goal” (Synder & Whitaker, 2013, p. 284). Expressing a general consensus, they argue that, after 1950, Penfield was seduced by artistry and simplicity, abandoning the “appropriate inductive approach” in the construction of future homunculi (p. 290). They accuse the 1954 sensorimotor homunculus of being “crudely drawn,” only loosely based on data, and 1 The classic book How to Lie with Statistics gives a very basic overview of this common error (Huff, 1954, p. 66). 2 I see the figures as two rather than (implicitly) three dimensional, but their perceived dimensionality is certainly debatable. If we adopt the three-dimensional perspective, the problem is even worse: The ratio of the inferred volumes of foot and finger is even greater than the ratio of their areas.
201
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Figure 2. Sketch of the homunculus by Penfield, probably c. 1951. ©Osler Library of the History of Medicine. Reproduced by permission of Osler Library of the History of Medicine, McGill University. Permission to reuse must be obtained from the rightsholder.
Downloaded by [University of Tasmania] at 09:46 02 September 2014
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Zina Ward
useless even as a mnemonic, asking “of what in memory these could [sic] be possibly be aiding, if all or most of the details of the drawings were not based on established functional anatomical correlates?” (p. 285). While I will not defend the 1954 thalamic homunculus, I do think that this assessment of the 1954 sensorimotor homunculus is far too harsh. The fact that it is indeed based on an analysis of Penfield’s data can be seen in its differences from the 1950 version, which reveal conscientious updating. First, there is the obvious addition of the secondary and supplementary areas: No longer was Penfield willing to ignore the important role played by extra-Rolandic cortex in somatic sensation and movement. Greater attention was also paid to lateralization in this homunculus. In the Rolandic cortex, only the right half of the body is shown since most responses elicited there were contralateral. In contrast, ipsilateral and bilateral responses were often observed in the secondary sensory and supplementary motor areas, which is why the figurines overlaid on these regions are whole (Penfield & Jasper, 1954, pp. 87, 103). The right half of both figurines is enlarged, however, reflecting the fact that contralateral responses were nevertheless still more common than ipsilateral ones. Other details in the image that reveal its careful, empirically constrained construction, such as the second sensory figurine’s exaggerated fingertips, are pointed out in the figure legend (Penfield & Jasper, 1954, p. 105). Snyder and Whitaker’s assertion that the 1954 sensorimotor homunculus is not even useful as a memory aid therefore seems extreme. It is based on Penfield’s stimulation data. It is visually engaging. It may be imprecise, with locations and sizes only approximately depicted, but precision is not required of a memory device. Indeed, one can hardly be expected to remember the exact lengths of the underlying lines in the 1950 homunculus; such details are superfluous when it comes to memorability. The 1954 version is therefore fully capable of fulfilling its stated purpose of aiding memory.3 I would like to offer a final suggestion that goes even further: I believe, contrary to the conclusion of Snyder and Whitaker and the prevailing consensus, that the 1954 homunculus is actually better than the 1950 version. This is because Penfield’s findings must be strongly qualified in several ways. First, his methods were crude and artificial, as many researchers and clinicians have since pointed out (Walshe, 1953; Schott, 1993; Schieber, 2001; Di Noto et al., 2012). Electrical stimulation — especially the strong stimulation used in Penfield’s era — is a poor imitation of genuine physiological processes, so, although it can be useful, one must be careful in drawing conclusions about normal functioning from it. Second, Penfield’s studies were all conducted on epileptics, raising concerns about generalizability. It is quite likely that there had been some degree of cortical reorganization following brain damage in most of Penfield’s patients, leading Snyder and Whitaker (and many others) to doubt the external validity of his results (2013, p. 288). The possibility of reorganization more generally — not just in pathological populations — also supports a cautious approach to claims about cortical organization. Both lesion- and use-dependent plasticity can alter the location and size of sensorimotor representations, so Penfield’s findings for each individual must not be considered final or fixed. Finally, Penfield’s stimulations often elicited responses from more than one body part at a time, as he and his coauthors amply acknowledge (e.g., Penfield & Boldrey, 1937, p. 430; Penfield & Rasmussen, 1950, p. 42). That different body parts did not have entirely distinct cortical representations is therefore an important caveat. These complications suggest that drawing any type of precise conclusion from Penfield’s studies about the location or size of sensorimotor representations would be a 3 In their concluding statement, Snyder and Whitaker seem to backpedal on this point, acknowledging the homunculi’s heuristic value (2013, p. 289).
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mistake. The cortical points that elicit somatic responses when strongly stimulated do not necessarily delineate a fixed region normally involved in the movement or sensation of those body part or parts. A scientific image summarizing Penfield’s findings ought not, then, convey exactitude. To do so would be to mislead the reader and to indulge in a false presumption to precision — in a sense, sacrificing accuracy for precision. The 1950 homunculus can be accused of this: Penfield and Rasmussen claim to have “corrected” the approximation of the figurine with precise underlying lines (1950, p. 214). The 1954 homunculus, in contrast, is cartoonish through-and-through: From the figurines’ expressions, to the scarcity of text, to the zoomed-out perspective, all aspects of the diagram suggest that it is meant to be merely schematic. This is not because, as Snyder and Whitaker suggest, artistic interest was a higher priority for Penfield than scientific accuracy. Rather, Penfield’s data did not warrant detailed, exact claims about cortical organization. The 1954 homunculus appropriately reflects that.
Acknowledgements My thoughts on the homunculus were shaped by time recently spent at the Osler Library in Montreal. Many thanks to the Mary Louise Nickerson Fellowship and those who helped me navigate the Penfield Archive, especially the late Dr. William Feindel.
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