Andrew F. Rossi

The representation of brightness in primary visual cortex

Although neurons in primary visual cortex are sensitive to the spatial distribution and intensity of light, their responses have not been thought to correlate with the perception of brightness. Indeed, primary visual cortex is often described as an initial processing stage that sends information to higher cortical areas where perception of brightness, color, and form occurs. However, a significant percentage of neurons in primary visual cortex were shown to respond in a manner correlated with perceived brightness, rather than responding strictly to the light level in the receptive fields of the cells. This finding suggests that even at the first stage of visual cortical processing, spatial integration of information yields perceptual qualities that are only indirectly related to the pattern of illumination of the retina.

The prefrontal cortex and the executive control of attention

We review two studies aimed at understanding the role of prefrontal cortex (PFC) in the control of attention. The first study examined which attentional functions are critically dependent on PFC by removing PFC unilaterally and transecting the forebrain commissures in two macaques. The monkeys fixated a central cue and discriminated the orientation of a colored target grating presented among colored distracter gratings in either the hemifield affected by the PFC lesion or the normal control hemifield. When the cue was held constant for many trials, task performance in the affected hemifield was nearly normal. However, performance was severely impaired when the cue was switched frequently across trials. The monkeys were unimpaired in a pop-out task with changing targets that did not require top-down attentional control. Thus, the PFC lesion resulted in selective impairment in the monkeys’ ability to switch top-down control. In the second study, we used fMRI to investigate the neural correlates of top-down control in humans performing tasks identical to those used in the monkey experiments. Several fronto-parietal and posterior visual areas showed enhanced activation when attention was switched, which was greater on color cueing (top-down) trials relative to pop-out trials. Taken together, our findings indicate that both frontal and parietal cortices are involved in generating top-down control signals for attentive switching, which may then be fed back to visual processing areas. The PFC in particular plays a critical role in the ability to switch attentional control on the basis of changing task demands.

Temporal Limits of Brightness Induction and Mechanisms of Brightness Perception

The luminance of a squarewave grating was modulated in a manner such that every other stripe temporally varied between bright and dark and the intervening stripes had constant luminance. This produces brightness induction in the constant stripes, roughly in antiphase to the luminance modulation. We used this stimulus as a probe to explore the temporal properties of brightness induction and the mechanisms determining perceived brightness. Over a range of spatial frequencies we measured: (1) the highest temporal frequency at which brightness induction occurs; (2) the magnitude of induced brightness; and (3) the temporal phase of the induced brightness modulation. We find that brightness induction ceases with luminance modulation above a cutoff temporal frequency that depends on spatial frequency. The magnitude of induced brightness modulation is greatest at low spatial frequencies and low temporal frequencies. Induced brightness lags behind the luminance modulation and this phase lag increases as spatial frequency decreases. All of these findings can be understood as consequences of an induction process that takes longer to complete as the induction region increases in size.

Top–Down Attentional Deficits in Macaques with Lesions of Lateral Prefrontal Cortex

Brain imaging, electrical stimulation, and neurophysiological studies have all implicated the prefrontal cortex (PFC) in the top–down control of attention. Specifically, feedback from PFC has been proposed to bias activity in visual cortex in favor of attended stimuli over irrelevant distracters. To identify which attentional functions are critically dependent on PFC, we removed PFC unilaterally in combination with transection of the corpus callosum and anterior commissure in two macaques. In such a preparation, the ipsilesional hemisphere is deprived of top–down feedback from PFC to visual cortex, and the contralesional hemisphere can serve as an intact normal control. Monkeys were trained to fixate a central cue and discriminate the orientation of a colored target grating presented among colored distracter gratings in either the hemifield affected by the PFC lesion or the normal control hemifield. Locations of the targets and distracters were varied, and the color of the central cue specified the color of the target on each trial. The behavioral response was a bar release, and thus attentional impairments could be distinguished from impaired oculomotor control. When the cue was held constant for many trials, task performance in the affected hemifield was nearly normal. However, the monkeys were severely impaired when the cue was switched frequently across trials. The monkeys were unimpaired in a pop-out task with changing targets that did not require top–down attentional control. The PFC thus appears to play a critical role in the ability to flexibly reallocate attention on the basis of changing task demands.

Feature-specific effects of selective visual attention

Four experiments were conducted to quantify the effect of performing a foveal discrimination task on sensitivity for a peripheral grating. The observers primary task was to discriminate either the spatial frequency or orientation of successive foveal Gabor patches. On a third of the trials they also performed a secondary task to detect the presence of a near-threshold grating in the periphery. We find that sensitivity for detection of the peripheral grating depends on the similarity of the spatial frequencies and orientations between the foveal and peripheral stimuli. Importantly, sensitivity is also affected by which feature is being discriminated in the central task. Because the detectability of the peripheral grating is different when different features of the central stimuli are discriminated, we suggest that the effects on sensitivity are due to feature-specific attention and not simply to passive interactions between filters with similar tuning properties.

The representation of brightness in primary visual cortex.

Although neurons in primary visual cortex are sensitive to the spatial distribution and intensity of light, their responses have not been thought to correlate with the perception of brightness. Indeed, primary visual cortex is often described as an initial processing stage that sends information to higher cortical areas where perception of brightness, color, and form occurs. However, a significant percentage of neurons in primary visual cortex were shown to respond in a manner correlated with perceived brightness, rather than responding strictly to the light level in the receptive fields of the cells. This finding suggests that even at the first stage of visual cortical processing, spatial integration of information yields perceptual qualities that are only indirectly related to the pattern of illumination of the retina.

Attentional control during the transient updating of cue information

The goal of the present study was to investigate the neural correlates of top-down control of switching behavior in humans and to contrast them to those observed during switching behavior guided by bottom-up mechanisms. In the main experimental condition (color-cue), which was guided by top-down control, a central cue indicated the color of a peripheral grating on which the subject performed an orientation judgment. For switch trials, the color of the cue on the current trial was different from the color on the previous trial. For non-switch trials, the color of the cue on the current trial was the same as the color in the preceding trial. During a control condition (pop-out), which was guided by bottom-up saliency, the target grating was defined by color contrast; again both switch and non-switch trials occurred. We observed stronger evoked responses during the color-cue task relative to the pop-out task in the inferior parietal lobule (IPL), frontal eye field (FEF), middle frontal gyrus (MFG), and inferior frontal gyrus (IFG). The contrast of switch vs. non-switch trials revealed activations in regions that were engaged when there was a change in the identity of the target. Collectively, switch trials evoked stronger responses relative to non-switch trials in fronto-parietal regions that appeared to be left lateralized, including left intraparietal sulcus (IPS) and left MFG/IFG. Task by trial type interactions (switch>non-switch during color-cue relative to pop-out) were observed in several fronto-parietal regions, including IPS, FEF, MFG and IFG, in addition to regions in visual cortex. Our findings suggest that, within the fronto-parietal attentional network, the IPS and MFG/IFG appear to be most heavily involved in attentive cue updating. Furthermore, several visual regions engaged by oriented gratings were strongly affected by cue updating, raising the possibility that they were the recipient of top-down signals that were generated when cue information was updated.