Session Detail

A recently discovered third receptor type, intrinsically photosensitive retinal ganglion cell (ipRGC), has attracted much attention because it affects circadian rhythm and sleep via signaling environmental light level. Nevertheless, few studies have examined whether ipRGCs also affect spatial vision. Here we investigate whether ipRGC stimulation in background affects human contrast sensitivity function (CSF), which varies with background luminance. A Gabor was presented at either left or right side of fixation. Participants were asked to perform a position-judgment task with the two-alternative forced-choice procedure. We measured CSFs at low spatial frequencies in periphery by adopting the silent substitution method to keep background color and luminance silent while manipulating ipRGC stimulation. Three conditions were tested: control, ipRGC-high, and light-flux-high condition. In ipRGC-high condition, only ipRGC stimulation was increased compared to control condition. In light-flux-high condition, both ipRGC and cone stimulations were increased. The results showed that increased ipRGC stimulation enhanced spatial contrast sensitivity while increased cone stimulation decreased it. Furthermore, increased ipRGC stimulation enhanced a spatial tuning in sensitivity according to changes in shape of the receptive field. Our findings indicate that, contrary to previous findings that ipRGCs contribute mainly to non-visual functions, ipRGCs also contributed to fundamental properties of spatial vision.

Unique hues are important as landmarks of the color appearance, while cone opponent mechanisms are physiologically important for coding colors in lower level of visual system. Considering the analogy with orientation selective neurons, more significant directions (e.g., horizontal and vertical) may be represented differently from others. We compared cortical responses to cardinal and unique hues under isoluminance using functional MRI. Subjects performed either hue-identification or letter memory (two-back) task in a run. Since identification of unique hues possibly employs feedbacks from higher order cortices, larger differences between color and letter tasks are expected with unique hues than with cardinal hues. The result on of GLM analysis on whole brain showed no significant difference between unique- and cardinal-hue responses, while comparison between letter- and color-task responses showed significant difference in frontal and occipital cortex. To examine differences in spatial pattern of brain activity between color and letter tasks, a representation similarity analysis using correlation coefficient was applied to V1 and V4 voxels. As a result, unique-hue responses showed significantly larger differences in both visual areas. This result implies that cortical representation of unique hues are affected more by feedback from higher order cortex than cardinal hues.

Deprive one specific orientation in two eyes’ viewings, for either a short period of 4 hours or a long period up to 4 days, has been shown to temporally enhance visual perception to the deprived orientation resulted from the regulation of visual adaptation. Here we show new evidence that if we conduct the orientation-deprivation monocularly for a short period of 2.5 hours in human adults, the deprived eye rather than the deprived orientation is strengthened in binocular viewing, and this effect could last at least 30 minutes. Such process is not orientation-selective and cannot be account for by adaptation. Thus, the present results suggest different form of the orientation deprivation-induced visual plasticity in human adults.

Orientation tuning in primary visual cortex (V1) neurons is contrast-invariant; while increasing contrast drives higher firing rates, orientation tuning bandwidths do not change. However, previous studies have not examined the time course of contrast adaptation, and whether stable orientation tuning is maintained as firing rates continually change. We recorded extracellular V1 activity in anaesthetised marmosets using two orientation reverse correlation paradigms, in which gratings with random orientation were updated every 16 ms. In the “slow adaptation” paradigm, four contrasts were tested, but contrast was fixed for 30 minutes. As shown previously, orientation tuning was contrast invariant. However, our ability to predict orientation from the neuronal responses using a linear decoder increased with contrast. In the “rapid adaptation” paradigm, luminance and contrast were changed every 5 seconds. Spiking rates increased immediately after contrast-increments, and then decayed exponentially. Our ability to decode orientation was highest when contrast was high, but surprisingly, within a period of constant contrast, spiking rate changed with no concomitant change in decoding performance. This dramatically demonstrates that adaptation within a few milliseconds allows neurons to encode orientation nearly independently contrast and firing rate; but on longer timescales, orientation encoding is contrast dependent, and correlated with mean spiking rate.

The marmoset (Callithrix jacchus) is emerging as a model for studies in visual neuroscience, but little is known about the frontal visual areas in this species. To date, the location of the marmoset frontal eye field (FEF) has only been suggested based on histology (part of cytoarchitectural areas 8aV and 45), anatomical connections with extrastriate cortex, and limited surface microstimulation. Here, we report on the results of experiments in which high-contrast flashing and moving stimuli were presented to 3 marmosets anaesthetized under opioid/ N2O anaesthesia, following implantation of 96-channel “Utah” arrays on the surface of the dorsolateral frontal cortex. The locations of the arrays were correlated with putative cytoarchitectural areas based on post-mortem MRI scan and registration to a marmoset brain template (Majka et al. J Comp Neurol 524: 2161-2181). We found that neurons with clear visual receptive fields were only found in areas 8aV, 8C and 6DR. The latency of the responses was 70-80 ms, compatible with the findings in macaque FEF. Only a minority of cells showed clear orientation and direction selectivity. Our findings provide a firm basis for further studies on the function of these visually-responsive regions in the context of attention and visual cognition.

The organization of the "third-tier" areas (Brodmann's area 19) in the primate visual cortex has been an issue of controversy. While the traditional view is that in small New World monkeys, adjoining the rostral border of dorsal V2 is the dorsomedial (DM) area, representing both the upper and the lower quadrants of the visaul field (Allman & Kaas, 1971), some researchers proposed a macaque-like area V3 representing only the lower quadrant. We densely mapped the visuotopy in this cortical region of the marmoset monkey, using 10-by-10 multi-electrode arrays, and were able to precisely duplicate the DM-map suggested by Rosa & Schmid (1995). Furthermore, we used white noise analysis to quantitatively model DM neurons' response characteristics, and showed that their responses could be approximated by small numbers of filters resembling Gabor functions, similar to the orientation-selective complex cells in V1. Finally, penetrations thought the medial segment of DM showed that although most DM neurons in the dorsal surface of DM are not direction selective, direction selectivity increases with eccentricity in DM. The results are consistent with the idea that DM is an intermediate-level area mediating functions related to both the dorsal and the ventral pathways.

Human neuroimaging studies have shown that the magnitude of orbitofrontal responses to neutral visual stimuli correlates with the strength of the preference for these stimuli obtained in behavioral studies. In the present study, we examined whether or not the magnitude of orbitofrontal single-neuron responses to neutral visual stimuli correlates with the strength of the preference for these stimuli observed in the behavioral study. First, we determined the strength of the preference (rank order) for 50 neutral visual stimuli behaviorally using two monkeys, and then examined correlations between the magnitude of orbitofrontal response to each of these stimuli and the behaviorally determined preference rank order of the stimulus. Among 188 neurons recorded, 65 exhibited responses to visual stimuli and exhibited stimulus selectivity. One third of these neurons exhibited either positive or negative correlations between the magnitude of visual responses and preference rank orders of visual stimuli. These results suggest that orbitofrontal neurons participate in the judgment of the preference for neutral visual stimuli. Since the orbitofrontal cortex is known to participate in the estimation of the value of the stimuli as a reward, preference judgment for neutral stimuli may correspond to this mechanism.