Function-specific projections from V2 to V4 in macaques

Direction-selective neurons in macaque V4

In mammalian visual system, direction-selective (DS) neurons prefer visual motion in a particular direction and are specialized for visual motion processing. In area V4 of the macaque, about 13% neurons are direction-selective and form clusters (DS domains). The functional role of DS neurons in this form-processing area is still unknown. We implanted electrode arrays targeting these DS domains and recorded neurons' responses to moving stimuli such as gratings and simple shapes. We found that DS neurons were similar to non-DS neurons in their receptive field sizes and orientation-selectivity properties. However, population-wise, DS neurons responded slower and had lower firing rates than non-DS neurons, contrary to their traditional role in motion processing. In addition, direction selectivity of V4 neurons was stimulus-dependent (i.e., not invariant). DS neurons identified with grating stimuli may not exhibit direction selectivity to other types of stimuli such as random dots or contour shapes. These results suggest that, unlike DS neurons in other areas, V4 DS neurons may have a unique origin for their direction selectivity and nontraditional roles in visual motion processing.NEW & NOTEWORTHY The functional role of direction-selective (DS) neurons in the ventral pathway is unclear. We studied DS neurons in area V4 of awake macaques. Interestingly, these neurons have slower responses and lower firing rates than those non-DS neurons. In addition, direction selectivity of these neurons was stimulus-type dependent. DS neurons in V4 may play a functional role different from those typical DS neurons in V1 or MT.

Orientation selectivity mapping in the visual cortex

The orientation map is one of the most well-studied functional maps of the visual cortex. However, results from the literature are of different qualities. Clear boundaries among different orientation domains and blurred uncertain distinctions were shown in different studies. These unclear imaging results will lead to an inaccuracy in depicting cortical structures, and the lack of consideration in experimental design will also lead to biased depictions of the cortical features. How we accurately define orientation domains will impact the entire field of research. In this study, we test how spatial frequency (SF), stimulus size, location, chromatic, and data processing methods affect the orientation functional maps (including a large area of dorsal V4, and parts of dorsal V1) acquired by intrinsic signal optical imaging. Our results indicate that, for large imaging fields, large grating stimuli with mixed SF components should be considered to acquire the orientation map. A diffusion model image enhancement based on the difference map could further improve the map quality. In addition, the similar outcomes of achromatic and chromatic gratings indicate two alternative types of afferents from LGN, pooling in V1 to generate cue-invariant orientation selectivity.

Mesoscale functional connectivity in macaque visual areas

Studies of resting-state functional connectivity (rsFC) have provided rich insights into the structures and functions of the human brain. However, most rsFC studies have focused on large-scale brain connectivity. To explore rsFC at a finer scale, we used intrinsic signal optical imaging to image the ongoing activity of the anesthetized macaque visual cortex. Differential signals from functional domains were used to quantify network-specific fluctuations. In 30-60 min resting-state imaging, a series of coherent activation patterns were observed in all three visual areas we examined (V1, V2, and V4). These patterns matched the known functional maps (ocular dominance, orientation, color) obtained in visual stimulation conditions. These functional connectivity (FC) networks fluctuated independently over time and exhibited similar temporal characteristics. Coherent fluctuations, however, were observed from orientation FC networks in different areas and even across two hemispheres. Thus, FC in the macaque visual cortex was fully mapped both on a fine scale and over a long range. Hemodynamic signals can be used to explore mesoscale rsFC in a submillimeter resolution.