Neurons responding selectively to different colours have been found in various cortical areas in macaque monkeys; however, little is known about whether and how the representation of colour is spatially organized in any cortical area. Cortical area V2 contains modules that respond preferentially to chromatic modulation, which are located in thin cytochrome oxidase stripes. Here we show that within and beyond these modules, gratings of different colours produce activations that peak at different locations. Optical recording of intrinsic signals revealed that the peak regions of the responses to different colours were spatially organized in the same order as colour stimuli are arranged in the DIN (German standard colour chart) colour system. Nearby regions represented colours of a similar hue. We found that the set of colour-specific regions formed 0.07-0.32-mm-wide and approximately 1.3-mm long bands that varied in shape from linear to nearly circular. Our finding suggests that thin stripes in V2 contain functional maps where the colour of a stimulus is represented by the location of its response activation peak.

The macaque striate cortex (V1) contains neurons that respond preferentially to various hues. The properties of these hue-selective neurons have been studied extensively at the single-unit level, but it is unclear how stimulus hue is represented by the distribution of activity across neuronal populations in V1. Here we use the intrinsic optical signal to image V1 responses to spatially uniform stimuli of various hues. We found that 1) Each of these stimuli activates an array of patches in the supragranular layers of the parafoveal V1; 2) The patches activated by different hues overlapped partially; 3) The peak locations of these patches were determined by stimulus hue. The peaks associated with various hues form well-separated clusters, in which nearby peaks represent perceptually similar hues. Each cluster represents a full gamut of hue in a small cortical area (∼160 μm long). The hue order is preserved within each peak cluster, but the clusters have various geometrical shapes. These clusters were co-localized with regions that responded preferentially to chromatic gratings compared with achromatic ones. Our results suggest that V1 contains an array of hue maps, in which the hue of a stimulus is represented by the location of the peak response to the stimulus. The orderly, organized hue maps in V1, together with the recently discovered hue maps in the extrastriate cortical area V2, are likely to play an important role in hue perception in primates.

To gain a deeper understanding of the transmission of visual signals from retina through the lateral geniculate nucleus (LGN), we have used a simple leaky integrate and-fire model to simulate a relay cell in the LGN. The simplicity of the model was motivated by two questions: (1) Can an LGN model that is driven by a retinal spike train recorded as synaptic ('S') potentials, but does not include a diverse array of ion channels, nor feedback inputs from the cortex, brainstem, and thalamic reticular nucleus, accurately simulate the LGN discharge on a spike-for-spike basis? (2) Are any special synaptic mechanisms, beyond simple summation of currents, necessary to model experimental recordings? We recorded cat relay cell responses to spatially homogeneous small or large spots, with luminance that was rapidly modulated in a pseudo-random fashion. Model parameters for each cell were optimized with a Simplex algorithm using a short segment of the recording. The model was then tested on a much longer, distinct data set consisting of responses to numerous repetitions of the noisy stimulus. For LGN cells that spiked in response to a sufficiently large fraction of retinal inputs, we found that this simplified model accurately predicted the firing times of LGN discharges. This suggests that modulations of the efficacy of the retino-geniculate synapse by pre-synaptic facilitation or depression are not necessary in order to account for the LGN responses generated by our stimuli, and that post-synaptic summation is sufficient.

The modular organization of cortical pathways linking visual area 4 ( V4) with occipital visual area 2 ( V2) and inferotemporal posterior inferotemporal ventral area (PITv) was investigated through an analysis of the patterns of retrogradely labeled cell bodies after injections of tracers into V4 and PITv. Although cytochrome oxidase or other stains have failed to yield reliable independent anatomical markers for cortical modules beyond V1 and V2, V4 and PITv seem to have modular compartments with specific patterns of cortico-cortical connectivity. Tracer injections of V4 labeled cells in V2 (1) thin stripes exclusively, (2) interstripes exclusively, or (3) specific combinations of interstripe and thin stripe subcompartments. These labeling patterns suggest (1) that there is a complicated organization of inputs to V4, (2) that projections from V2 to V4 display a submodular selectivity, and (3) that projections from V2 to V4 display some degree of cross-stream convergence. Consistent with this framework, extensive regions of PITv provide feedback projections to interstripe-recipient portions of V4, whereas more restricted portions of PITv provide feedback to thin stripe-recipient portions of V4. Similarly, the feedforward projection from V4 to PITv often arose from multiple cell clusters across a wide expanse of V4. When distinguishable fluorescent tracers were injected into two PITv sites separated by 3-5 mm, a variety of projection patterns was observed in V4. In most cases, labeled cells were found in multiple, interdigitating, nonoverlapping clusters of 1-3 mm width, whereas in other cases the two labeled fields were highly intermixed. These results suggest that V4 and PITv contain functional modules that can be characterized by the specific patterns of segregated and convergent projections they receive from lower cortical areas. These specific patterns of intercortical input, in conjunction with intrinsic cortical circuitry, may endow extrastriate cortical neurons with new and more complex receptive field properties. Key words: functional architecture; temporal lobe; cortical pathways; retrograde tracers; cytochrome oxidaseThe supragranular layers of visual area 1 (V1) are characterized by cytochrome oxidase dense blobs surrounded by pale interblobs (for example, see Livingstone and Hubel, 1984), whereas V2 is characterized by alternating sets of thick and thin dense stripes separated by pale stripes (for example, see Hubel and Livingstone, 1987). Functional studies have contributed to the notion that V1 consists of three compartments associated with parallel functional streams: the blob-dominated stream (BD) involved in color analysis, the interblob-dominated stream (IB) involved in shape analysis, and the magnocellular-dominated stream (MD), which arises from layer 4B and contains orientation and direction-selective cells that receive input from magnocellular LGN recipient layer 4C␣ (Van Essen and Gallant, 1994). These three functional streams target specific compartments in V2: the cytochrome oxidase dens...