Functional

visual

streams

John H. R. Maunsell University

of Rochester,

Rochester,

The idea that visual signals relayed subdivisions cerebral

of the

cortex

parvocellular

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that

experiments

shown

pathways

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that

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It has been proposed

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by the parvocellular

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New York,

in the

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of primate

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is not this simple.

in Neurobiology

Introduction

1992, 2:50&510

Distinctions

between

the parietal

and temporal

pathways The visual system separates different types of information into parallel anatomically segregated streams of processing. In primates the tracts from the retina to the primary visual cortex (Vl) are cleanly split into pathways that are relayed by the magnocellular (M) and parvocellular (P) subdivisions of the lateral geniculate nucleus (LGN). The M and P pathways differ markedly along several anatomical and physiological dimensions (see [ l-31 ). The higher levels of visual cerebral cortex also contain two streams of processing, each of which includes multiple visual areas and mediates different visual behaviors [4]. A dorsal stream includes areas in the parietal cortex and is important for vision related to motion or spatial relationships, while a ventral stream includes visual areas in the temporal lobe and is more involved in form analysis and object recognition (see [5]). Converging observations from anatomical, physiological and behavioral experiments led to the proposal that a direct correspondence exists between these subcortical and cortical pathways [ 6,7]. It was suggested that the panetal pathway received most of its excitatory drive from the M pathway, while the P pathway contributions dominated cortical areas in the temporal pathway. The possibility that parallel streams originate in the retina and operate more-or-less independently up to the highest levels of visual cortex (Fig.1) has far-reaching implications for understanding the functional organization of the visual system and the nervous system in general. Although the original proposal was based on a large body of observations, the data were not conclusive and left open distinctly different interpretations (see [8]). Work in the intervening years has on the one hand strengthened the case for parallel pathways in higher visual cortex, and on the other hand failed to establish a one-toone link between subcortical and cortical pathways. This review will focus on the results of these studies.

Several recent reports have supported a distinction between the parietal and temporal pathways in primate visual cortex. Two anatomical studies have shown that sites in the parietal and temporal pathways in monkeys receive input from largely non-overlapping regions of visual cortex. Morel and Bullier [9], and Baizer and her colleagues [lo-] performed similar studies in which they injected different tracers into parietal and inferotemporal cortex. Although each tracer labeled neurons over extensive regions of cortex, there was relatively little overlap in the distributions. These studies provide some of the most direct anatomical demonstrations of separate streams of processing in the cortex. Other studies, using positron emission tomography (PET), have shown different functional contributions of parietal and temporal cortex in humans. Haxby and his coworkers [ll-] asked subjects to perform either a face matching task or a spatial localization task and found that temporal cortex was preferentially activated during the former, while parietal cortex was more active during the latter. Zeki et al. [12**] have shown a similar separation of cortical activation in subjects viewing colored or moving stimuli. Corbetta and colleagues [ 13**] have examined changes in cortical activity related to differences in attention rather than to different stimuli. In their study subjects always viewed the same stimuli, but at different times were required to attend to either the speed, the shape, or the color of the stimuli. Attention to speed activated regions in the inferior parietal lobule and superior temporal sulcus, while attention to shape or color resulted in more activity in the inferior temporal lobe, although the separation of the activated regions was not as pronounced as in the studies that used different visual stimuli. Increasingly refined testing of patients with cortical damage also supports a distinction between the visual contributions of temporal and parietal cortex [ 14,151.

Abbreviations LCN-lateral

506

geniculate nucleus; M-magnocellular; MT-middle temporal visual area; P-parvocellular; PI!‘-positron emission tomography; Vl-primary visual cortex. @ Current

Biology

Ltd ISSN 0959-4383

Functional

Parietal pathway

visual

streams

Maunsell

Dominance of MT by M pathway contributions has since been supported by many observations [ 2,0,21], including recordings made in MT while selectively inactivating the M or P pathways at the level of the LGN [ 221. Visual responses in MT are always reduced or eliminated when the M pathway is blocked, and rarely affected when the P pathway is inactivated.

Temporal pathway

The suggestion that the temporal pathway might be dominated by P pathway contributions has always had a weaker foundation, and recent results suggest that the temporal pathway receives strong inputs from both the P and the M pathway. The picture that is emerging is one in which there is only a partial correspondence between the subcortical and cortical pathways. Evidence for M and P contributions temporal

I

Parvo

I

77 P cells

M

pathway

P pathway

Fig.1. Parallel organization

in the primate visual system. The projection from the retina to the subdivisions of layer 4C in VI is sharply divided into magnocellular (MI and parvocellular (P) pathways. Processing in higher visual cerebral cortex can also be divided into parietal and temporal pathways, although crossconnections between the two cortical pathways have been demonstrated. It had been proposed that anatomical subdivisions within VI and V2 might serve to maintain a segregation between M and P pathway contributions and direct them selectively to the parietal and temporal pathways. Recent studies have shown that M and P pathway contributions mix extensively within VI. This figure shows a highly selected subset of the established areas and connections in the monkey visual system. Adapted from [18*1.

Together with a large body of earlier work, these studies make a strong case for functionally distinct par&al and temporal pathways that contribute differentially to the spatial/motion and form/color aspects of vision. Our understanding of their exact roles is far from complete and alternative interpretations of their functional contnbutions exist [16,17*]. While the parietal and temporal pathways clearly interact extensively (see [18*,19]), the available evidence suggests that their distinctions are more than sufficient to justify viewing them as separate streams of processing. Crossed connections The idea that the parietal and temporal streams might be continuations of the M and P pathways was based in large part on the M pathway’s apparent dominance in the middle temporal visual area (MT), which is an irnportant component of the parietal pathway (see [ 5-71).

in the

pathway

There has been good reason to expect strong P pathway contributions to the temporal pathway. The neurons in layer 4Cp in cortical area Vl, upon which the LGN parvocellular projection terminates, send their axons directly to neurons in both the cytochrome oxidase blobs and the interblobs in the superficial layers (Fig.1). These in turn lead to the thin stripes and interstripes in V2, and from there to V4, which lies in the temporal pathway. What has been lacking is evidence that M pathway contributions are excluded from these structures. Some previous observations suggested that they are not. Neurons in layers 4Ca and 4B (M pathway) send substantial projections to the superficial layers of Vl [ 23,241. Furthermore, cortical recordings during selective inactivation of either the P or M pathway showed that about 40 % of VI neurons received excitatory drive from both pathways [ 251. More recent studies have added weight to the argument that M pathway contributions are well represented in the temporal pathway. Lachica, Beck and Casagrande [26**] examined the distributions of labeled neurons in layer 4C after injecting tracers into the blobs or the interblobs in superficial Vl. They confirmed that the M pathway layers 4Ca and 4B send substantial projections to the superficial layers of Vl, and found that these projections are confined to the qtochrome oxidase blobs. Selective inactivation of the P and M pathways coupled with histological reconstruction of recording sites suggests that M pathway contributions find their way to the interblobs as well, and that they are comparable to P pathway contributions throughout the superficial layers of Vl [ 27**]. Contributions from the M pathway might reach neurons in the interblobs by way of extensive horizontal connections that exist in cortex [28], or through feedback connections from other cortical areas [ 29=]. Another hint of an association between the M pathway and the interblob regions is seen in the distribution of Meynert cells. These cells, which project to the superior colliculus and MT (both of which are dominated by M pathway contributions [ 22,30]), are preferentially distributed beneath the interblobs in the superficial layers [ 31,321. An M pathway contribution has also been demonstrated in later stages of the temporal pathway. Recordings from V4 during selective M pathway or P pathway inactivation demonstrate

507

508

Sensory systems

M pathway contributions of similar strength to those seen in the blobs and interblobs [33-l. In conclusion, the available anatomical and physiological data provide no evidence that the temporal pathway is dominated by the P pathway. This differs from the panetal pathway, which appears to be dominated by M pathway contributions [ 221. Thus, the relationships between the subcortical and cortical pathways appear to be asym metric. Behavioral

observations

One of the lines of evidence for separate contributions from the M and P pathways to the parietal and temporal pathways came from psychophysical measurements made under conditions that were thought to activate either the M pathway or the P pathway in isolation. The weight of these arguments has been reduced by recent work showing that these approaches do not effectively isolate either pathway. The P pathway carries almost all information about color that is provided to the cortex [2]. Because stereopsis, motion, and form and pattern perception were all degraded or eliminated using isoluminant color stimuli, it was proposed that these functions could be attributed to the M pathway [7]. Recent reports have pointed out flaws in this approach [34,35]. Although the M pathway is relatively unresponsive to isoluminant chromatic stimuli, the performance of the P pathway is degraded as well [34,36,37]. Thus, reduced behavioral performance using isoluminant color stimuli might result from suboptimal activation of either the M pathway or the P pathway or both. Evidence that the P pathway does in fact contribute to the perception of form and depth comes from experiments that show that these properties are readily perceived in after-images, which should preferentially activate the P pathway [37]. A further complication arising from the use of isoluminant color stimuli is that neurons in the M pathway are not silent at isoluminance. Although they fail to distinguish colors, they do respond to color borders ([36,38,39], and see [40*]). These considerations leave little basis for attributing the capabilities lost at isoluminance to selective inactivation of the M pathway. Other problems with this general approach are seen in attempts to activate the M pathway selectively using low contrast stimuli 171. Although neurons in the P pathway are far less sensitive to low contrasts [41], there is no reason to expect that they are completely unresponsive to such stimuli. Given that P cells in the retina outnumber M cells eight to one [42,43], their activity might be summed in the cortex to support perceptions at low contrasts [44-l. Thus there is little basis for attributing performance at low contrast to contributions of the M pathway. Selective

lesions of the M and P pathways

Some of the clearest evidence about the role of the M and P pathways in the analysis of different visual attributes comes from recent studies in which the processing of

either pathway is disrupted by small localized lesions in the M or P subdivisions of the LGN. For the most part, the effects of such lesions are best described in terms of low-level visual properties rather than the higher-level functions associated with the parietal and temporal pathways. Lesions of the P subdivision reduce acuity by an amount that closely matches predictions based on the differences in the density of P and M ganglion cells in the retina [45**]. P lesions also result in a complete loss of color vision (45..,461, a result consistent with the absence of color-opponency in the M pathway. P lesions reduce behavioral performance at high spatial or low temporal frequencies, ranges within which the M pathway neurons show reduced sensitivity [2,47]. On the other hand, several higher-level functions, including shape discrimination and stereopsis, appear unaffected by P pathway lesions, except when they depend on high spatial or low temporal frequencies [46,48-l. The effects of M pathway lesions are also best described in terms of low-level properties. In contrast to the P pathway lesions, disruption of the M pathway reduces sensitivity to low spatial frequencies and high temporal frequencies, and does not affect acuity or color vision [45*-,46,49-l. Animals with M pathway lesions can discriminate directions of movement [ 49**], a result consistent with psychophysical observations which suggest that the P pathway is able to support motion vision ([50-l, and see [40-l ). M pathway lesions do not affect stereopsis [46], showing that the P pathway alone can mediate stereopsis. Interestingly, isoluminant stimuli profoundly degrade stereopsis, again suggesting that such stimuli do not fully reveal the capabilities of the P pathway. Conclusions Recent experiments have shown that the correspondence between the M and P pathways and the parietal and temporal pathways is not one-to-one. The available evidence, while far from complete, provides no compelling reason to believe that M and P pathway contributions do not have equal access to the temporal pathway. On the other hand, it would probably be wrong to claim that no direct relationships exist between the cortical and subcortical pathways. A large set of anatomical and physiological evidence shows that the parietal pathway is dominated by M pathway contributions. An alternative view of visual system organization is that the relationships between the subcortical and cortical pathways are largely incidental and that the two subcortical pathways are simply an efficient way to encode spatial and temporal information (see [ 191). With this view, the cortical pathways draw from the subcortical pathways according to the needs of the functions they perform. The analysis of motion in the parietal pathway would depend on high temporal and low spatial frequencies because these values distinguish moving stimuli. Lesser contributions would be taken from the P pathway to contribute to the limited subset of motion signals that are best represented by that pathway (for examples, see [51*,52] ). The analysis of color and form in the temporal

Functional

pathway draws information from the P pathway. Within the framework of this view, it is presumed that the signals relayed by the M pathway are also useful for the form and object recognition mediated by the temporal pathway. This interpretation of the relationships between the subcortical and cortical pathways is difficult to test. Until much more is understood about visual processing it will be impossible to defend or refute the idea that a particular type of signal is, or is not, suited to a particular visual function. Nevertheless, the results of the investigations summarized here show clearly that higher visual functions leading to the perception of spatiotemporal relationships or to visual object recognition do not depend exclusively on information processed by either the M or the P pathways. Acknowledgements Preparation of this review was supported by NIH EY05911, ONR N00014-9@J-1070, and a McKnight Neuroscience Development Award. RA Eatock, NK Logothetis and TA Nealey provided valuable comments on earlier versions of the manuscript.

and recommended

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