Henk Spekreijse

Object-based attention in the primary visual cortex of the macaque monkey.

Typical natural visual scenes contain many objects, which need to be segregated from each other and from the background. Present theories subdivide the processes responsible for this segregation into a pre-attentive and attentive system,. The pre-attentive system segregates image regions that ‘pop out’ rapidly and in parallel across the visual field. In the primary visual cortex, responses to pre-attentively selected image regions are enhanced. When objects do not segregate automatically from the rest of the image, the time-consuming attentive system is recruited. Here we investigate whether attentive selection is also associated with a modulation of firing rates in area V1 of the brainin monkeys trained to perform a curve-tracing task,. Neuronal responses to the various segments of a target curve were simultaneously enhanced relative to responses evoked by a distractor curve, even if the two curves crossed each other. This indicates that object-based attention is associated with a response enhancement at the earliest level of the visual cortical processing hierarchy.

Feedforward, horizontal, and feedback processing in the visual cortex.

The cortical visual system consists of many richly interconnected areas. Each area is characterized by more or less specific receptive field tuning properties. However, these tuning properties reflect only a subset of the interactions that occur within and between areas. Neuronal responses may be modulated by perceptual context or attention. These modulations reflect lateral interactions within areas and feedback from higher to lower areas. Recent work is beginning to unravel how horizontal and feedback connections each contribute to modulatory effects and what the role of these modulations is in vision. Whereas receptive field tuning properties reflect feedforward processing, modulations evoked by horizontal and feedback connections may reflect the integration of information that underlies perception.

Two distinct modes of sensory processing observed in monkey primary visual cortex (V1).

Even salient sensory stimuli are sometimes not detected. What goes wrong in the brain in that case? Here we show that a late (> 100-ms) component of the neural activity in the primary visual cortex of the monkey is selectively suppressed when stimuli are not seen. As there is evidence that this activity depends on feedback from extrastriate areas, these findings suggest a specific role for recurrent processing when stimuli are reaching a perceptual level. Further results show that this perceptual level is situated between purely sensory and decision or motor stages of processing.

Contrast evoked responses in man

Abstract In man visually evoked responses (EPs) can be recorded that are specific to changes in spatial contrast and cannot be derived from luminance responses. No spatial contrast component could be demonstrated in the ERG. Contrast EPs depend on various parameters such as acuity, retinal location, size and configuration of spatial elements, time course of luminance change. They are affected by overlapping steady contrasts presented either monocularly or dichoptically. For a given condition the contrast EP is mainly determined by the instantaneous relative contrast irrespective whether this contrast is reached by an increase or decrease in luminance. The EPs to the appearance and disappearance of a pattern bear different relations to stimulus parameters and seem to originate from different populations of cortical cells. The responses to short appearances obey the contrast equivalent of Blochs law and correlate with psychophysics.

Principal components analysis for source localization of VEPs in man

This study defines and compares the topologies of the visual evoked potentials to various stimuli such as pattern onset/offset, pattern reversal, pattern motion and high frequency luminance flicker. The responses recorded from 24 occipital derivations were examined using a three sphere conductance model to represent the head, with the assumption that activity from an underlying cortical source is equivalent to a single dipole. Principal components analysis was used to find the dimensionality of the data space. From this analysis could be concluded that all stimuli evoked responses in the primary visual cortex. Only pattern onset, and to a lesser degree pattern offset and pattern reversal, yielded activity in higher visual areas. In particular it has been shown that the CI, CII interval of the pattern onset response has its origins in two different cortical regions. A fast positive (CI)-negative (part of the CII) component arises from area 18 (or 19), a slower negative (initial part of CII) component comes from area 17.

Masking Interrupts Figure–Ground Signals in V1

In a backward masking paradigm, a target stimulus is rapidly (<100 msec) followed by a second stimulus. This typically results in a dramatic decrease in the visibility of the target stimulus. It has been shown that masking reduces responses in V1. It is not known, however, which process in V1 is affected by the mask. In the past, we have shown that in V1, modulations of neural activity that are specifically related to figure-ground segregation can be recorded. Here, we recorded from awake macaque monkeys, engaged in a task where they had to detect figures from background in a pattern backward masking paradigm. We show that the V1 figure-ground signals are selectively and fully suppressed at target-mask intervals that psychophysically result in the target being invisible. Initial response transients, signalling the features that make up the scene, are not affected. As figure-ground modulations depend on feedback from extrastriate areas, these results suggest that masking selectively interrupts the recurrent interactions between V1 and higher visual areas.

Mathematical dipoles are adequate to describe realistic generators of human brain activity

It is investigated whether a mathematical dipole description is adequate for the localization of brain activity on the basis of visually evoked potentials (VEPs). Extended sources (dipole disks and dipole annuli) are stimulated and fitted with a mathematical dipole. It is found that the deviation between the positions of the disks and annuli and the equivalent dipole is very small. Also, the differences in the direction and amplitude may be neglected. The position of the extended source with respect to the electrode grid does not much influence these conclusions.<<ETX>>

Figure–Ground Segregation in a Recurrent Network Architecture

Here we propose a model of how the visual brain segregates textured scenes into figures and background. During texture segregation, locations where the properties of texture elements change abruptly are assigned to boundaries, whereas image regions that are relatively homogeneous are grouped together. Boundary detection and grouping of image regions require different connection schemes, which are accommodated in a single network architecture by implementing them in different layers. As a result, all units carry signals related to boundary detection as well as grouping of image regions, in accordance with cortical physiology. Boundaries yield an early enhancement of network responses, but at a later point, an entire figural region is grouped together, because units that respond to it are labeled with enhanced activity. The model predicts which image regions are preferentially perceived as figure or as background and reproduces the spatio-temporal profile of neuronal activity in the visual cortex during texture segregation in intact animals, as well as in animals with cortical lesions.

Horizontal Cells Feed Back to Cones by Shifting the Cone Calcium-Current Activation Range

We studied feedback from horizontal cells to cones in isolated goldfish retinae and found that surround stimuli evoke an inward current and a slowly developing outward current. The surround-evoked currents are blocked by the glutamate antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX) and are, like horizontal cell responses, most effectively evoked by large stimuli. This indicates that the currents are caused by feedback from horizontal cells. The surround-evoked inward current is neither blocked by picrotoxin nor carried by chloride. Instead, it is carried by calcium, and it triggers a slowly developing calcium-dependent chloride current. We were unable to mimick the surround-evoked currents by modulating the extracellular GABA concentration. We conclude that when horizontal cells hyperpolarize they feed back to the cones by shifting the cone calcium-current activation range to more negative potentials. This type of feedback, directly targeted at the calcium current, scarcely influences the membrane potential of the receiving neuron, but effectively modulates its synaptic output.

Synchrony and covariation of firing rates in the primary visual cortex during contour grouping.

The visual system imposes structure onto incoming information, by grouping image elements of a single object together, and by segregating them from elements that belong to other objects and the background. One influential theory holds that the code for grouping and segmentation is carried by the synchrony of neuronal discharges on a millisecond time scale. We tested this theory by recording neuronal activity in the primary visual cortex (area V1) of monkeys engaged in a contour-grouping task. We found that synchrony was unrelated to contour grouping. The firing rates of V1 neurons are also correlated across trials. We demonstrate that this rate covariation is mainly determined by fluctuations in visual attention. Moreover, we show that rate covariation depends on perceptual grouping, as it is strongest between neurons that respond to features of the same object.