In recent years, many new cortical areas have been identified in the macaque monkey. The number of identified connections between areas has increased even more dramatically. We report here on (1) a summary of the layout of cortical areas associated with vision and with other modalities, (2) a computerized database for storing and representing large amounts of information on connectivity patterns, and (3) the application of these data to the analysis of hierarchical organization of the cerebral cortex. Our analysis concentrates on the visual system, which includes 25 neocortical areas that are predominantly or exclusively visual in function, plus an additional 7 areas that we regard as visual-association areas on the basis of their extensive visual inputs. A total of 305 connections among these 32 visual and visual-association areas have been reported. This represents 31% of the possible number of pathways if each area were connected with all others. The actual degree of connectivity is likely to be closer to 40%. The great majority of pathways involve reciprocal connections between areas. There are also extensive connections with cortical areas outside the visual system proper, including the somatosensory cortex, as well as neocortical, transitional, and archicortical regions in the temporal and frontal lobes. In the somatosensory/motor system, there are 62 identified pathways linking 13 cortical areas, suggesting an overall connectivity of about 40%. Based on the laminar patterns of connections between areas, we propose a hierarchy of visual areas and of somatosensory/motor areas that is more comprehensive than those suggested in other recent studies. The current version of the visual hierarchy includes 10 levels of cortical processing. Altogether, it contains 14 levels if one includes the retina and lateral geniculate nucleus at the bottom as well as the entorhinal cortex and hippocampus at the top. Within this hierarchy, there are multiple, intertwined processing streams, which, at a low level, are related to the compartmental organization of areas V1 and V2 and, at a high level, are related to the distinction between processing centers in the temporal and parietal lobes. However, there are some pathways and relationships (about 10% of the total) whose descriptions do not fit cleanly into this hierarchical scheme for one reason or another. In most instances, though, it is unclear whether these represent genuine exceptions to a strict hierarchy rather than inaccuracies or uncertainities in the reported assignment.

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Distributed Hierarchical Processing in the Primate Cerebral Cortex

The two basic phenomena that define the problem of visual attention can be illustrated in a simple example. Consider the arrays shown in each panel of Figure 1. In a typical experiment, before the arrays were presented, subjects would be asked to report letters appearing in one color (targets, here black letters), and to disregard letters in the other color (nontargets, here white letters). The array would then be briefly flashed, and the subjects, without any opportunity for eye movements, would give their report. The display mimics our. usual cluttered visual environment: It contains one or more objects that are relevant to current behavior, along with others that are irrelevant. The first basic phenomenon is limited capacity for processing information. At any given time, only a small amount of the information available on the retina can be processed and used in the control of behavior. Subjectively, giving attention to any one target leaves less available for others. In Figure 1, the probability of reporting the target letter N is much lower with two accompa­ nying targets (Figure la) than with none (Figure Ib). The second basic phenomenon is selectivity-the ability to filter out un­ wanted information. Subjectively, one is aware of attended stimuli and largely unaware of unattended ones. Correspondingly, accuracy in identifying an attended stimulus may be independent of the number of nontargets in a display (Figure la vs Ie) (see Bundesen 1990, Duncan 1980).

Neural Mechanisms of Selective Visual Attention

Separate visual pathways for perception and action.

The ionotropic glutamate receptors are ligand-gated ion channels that mediate the vast majority of excitatory neurotransmission in the brain. The cloning of cDNAs encoding glutamate receptor subunits, which occurred mainly between 1989 and 1992 ([Hollmann and Heinemann, 1994][1]), stimulated this

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The glutamate receptor ion channels

Neuronal Synchrony: A Versatile Code for the Definition of Relations?

Many lines of evidence suggest that the visual signals relayed through the magnocellular and parvocellular subdivisions of the primate dorsal LGN remain largely segregated through several levels of cortical processing. It has been suggested that this segregation persists through to the highest stages of the visual cortex, and that the pronounced differences between the neuronal response properties in the parietal cortex and inferotemporal cortex may be attributed to differential contributions from magnocellular and parvocellular signals. We have examined this hypothesis directly by recording the responses of cortical neurons while selectively blocking responses in the magnocellular or parvocellular layers of the LGN. Responses were recorded from single units or multiunit clusters in the middle temporal visual area (MT), which is part of the pathway leading to parietal cortex and thought to receive primarily magnocellular inputs. Responses in the MT were consistently reduced when the magnocellular subdivision of the LGN was inactivated. The reduction was almost always pronounced and often complete. In contrast, parvocellular block rarely produced striking changes in MT responses and typically had very little effect. Nevertheless, unequivocal parvocellular contributions could be demonstrated for a minority of MT responses. At a few MT sites, responses were recorded while magnocellular and parvocellular blocks were made simultaneously. Responses were essentially eliminated for all these paired blocks. These results provide direct evidence for segregation of magnocellular and parvocellular contributions in the extrastriate visual cortex and support the suggestion that these signals remain largely segregated through the highest levels of cortical processing.

https://www.jneurosci.org/content/jneuro/10/10/3323.full.pdf

Magnocellular and parvocellular contributions to responses in the middle temporal visual area (MT) of the macaque monkey

Several neurophysiological studies have shown that the visual cerebral cortex of macaque monkeys performing delayed match-to-sample tasks contains individual neurons whose levels of activity depend on the sample the animal is required to remember. Haenny et al. (1988) reported that the activity of neurons in area V4 of monkeys performing an orientation matching task depends on the orientation for which the animal is searching. It was proposed that these neurons contribute to a representation of the orientation being sought. We have further characterized these neurons by recording visual responses from individual neurons during multiple behavioral tasks. Animals were trained to perform an orientation match-to-sample task using either a visual or a tactile orientation sample. In a set of 89 neurons examined using both types of sample, 25% showed statistically significant effects of sample orientation regardless of whether the sample was visual or tactile. Most of these preferred the same sample orientation in both conditions. These results allow us to specify the nature of the information signaled by these neurons more precisely than has previously been possible. For 193 units tested using one of the matching tasks, responses were also recorded while the animal performed a simple fixation task. In this task the animal was not required to attend to the visual stimuli that were presented. A few neurons that were responsive during the matching task were silent during fixation, but a comparable number was much more responsive during fixation. Across the whole population there was no systematic change in either responsivity or selectivity for orientation under the two conditions.

Extraretinal representations in area V4 in the macaque monkey.

A substantial body of evidence has suggested that signals transmitted through the magnocellular and parvocellular subdivisions of the LGN remain largely segregated in visual cortex. This hypothesis can be tested directly by selectively blocking transmission through either the magnocellular or parvocellular layers with small injections of lidocaine or GABA while recording cortical responses to a visual stimulus. In a previous study, we found that responses in the middle temporal visual area (MT) were almost always greatly reduced by blocks of magnocellular LGN, but that few MT neurons were affected by parvocellular blocks. In the present study, we have examined magnocellular and parvocellular contributions to area V4, which lies at the same level of processing in the cortical hierarchy as does MT and is thought to be a major recipient of parvocellular input. We found that inactivation of parvocellular layers usually resulted in a moderate reduction of visual responses (median reduction, 36%). However, comparable reductions in V4 responses were also seen following magnocellular blocks (median reduction, 47%). Directionally selective responses in V4 were not found to depend specifically on either subdivision. We conclude that area V4, unlike MT, receives strong input from both subdivisions of the LGN. These results suggest that the relationship between the subcortical magnocellular and parvocellular pathways and the parietal and temporal streams of processing in cortex is not one-to-one.

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Responses in macaque visual area V4 following inactivation of the parvocellular and magnocellular LGN pathways

Anatomical and physiological studies of the primate visual system have suggested that the signals relayed by the magnocellular and parvocellular subdivisions of the LGN remain segregated in visual cortex. It has been suggested that this segregation may account for the known differences in visual function between the parietal and temporal cortical processing streams in extrastriate visual cortex. To test directly the hypothesis that the temporal stream of processing receives predominantly parvocellular signals, we recorded visual responses from the superficial layers of V1 (striate cortex), which give rise to the temporal stream, while selectively inactivating either the magnocellular or parvocellular subdivisions of the LGN. Inactivation of the parvocellular subdivision reduced neuronal responses in the superficial layers of V1, but the effects of magnocellular blocks were generally as pronounced or slightly stronger. Individual neurons were found to receive contributions from both pathways. We furthermore found no evidence that magnocellular contributions were restricted to either the cytochrome oxidase blobs or interblobs in V1. Instead, magnocellular signals made substantial contributions to responses throughout the superficial layers. Thus, the regions within V1 that constitute the early stages of the temporal processing stream do not appear to contain isolated parvocellular signals. These results argue against a direct mapping of the subcortical magnocellular and parvocellular pathways onto the parietal and temporal streams of processing in cortex.

https://www.jneurosci.org/content/jneuro/14/4/2069.full.pdf

Magnocellular and parvocellular contributions to the responses of neurons in macaque striate cortex

Visual information from the retina is transmitted to the cerebral cortex by way of the lateral geniculate nucleus (LGN) in the thalamus. In primates, most of the retinal ganglion cells that project to the LGN belong to one of two classes, P and M, whose axons terminate in the parvocellular or magnocellular subdivisions of the LGN. These cell classes give rise to two channels that have been distinguished anatomically, physiologically and behaviourally. The visual cortex also can be subdivided into two pathways, one specialized for motion processing and the other for colour and form information. Several lines of indirect evidence have suggested a close correspondence between the subcortical and cortical pathways, such that the M channel provides input to the motion pathway and the P channel drives the colour/form pathway. This hypothesis was tested directly by selectively inactivating either the magnocellular or parvocellular subdivision of the LGN and recording the effects on visual responses in the cortex. We have previously reported that, in accordance with the hypothesis, responses in the motion pathway in the cortex depend primarily on magnocellular LGN. We now report that in the colour/form pathway, visual responses depend on both P and M input. These results argue against a simple correspondence between the subcortical and cortical pathways.

Tara A. Nealey

Papers

Magnocellular and parvocellular contributions to responses in the middle temporal visual area (MT) of the macaque monkey

Extraretinal representations in area V4 in the macaque monkey.

Responses in macaque visual area V4 following inactivation of the parvocellular and magnocellular LGN pathways

Magnocellular and parvocellular contributions to the responses of neurons in macaque striate cortex

Mixed parvocellular and magnocellular geniculate signals in visual area V4.