Angela L. Gee

Activity in the Lateral Intraparietal Area Predicts the Goal and Latency of Saccades in a Free-Viewing Visual Search Task

The purpose of saccadic eye movements is to facilitate vision, by placing the fovea on interesting objects in the environment. Eye movements are not made for reward, and they are rarely restricted. Despite this, most of our knowledge about the neural genesis of eye movements comes from experiments in which specific eye movements are rewarded or restricted. Such experiments have demonstrated that activity in the lateral intraparietal (LIP) area of the monkey correlates with the monkeys planning of a memory-guided saccade or deciding where, on the basis of motion information, to make a saccade. However, other experiments have shown that neural activity in LIP can easily be dissociated from the generation of saccadic eye movements, especially when sophisticated behavioral paradigms dissociate the monkeys locus of attention from the goal of an intended saccade. In this study, we trained monkeys to report the results of a visual search task by making a nontargeting hand movement. Once the task began, the monkeys were entirely free to move their eyes, and rewards were not contingent on the monkeys making specific eye movements. We found that neural activity in LIP predicted not only the goal of the monkeys saccades but also their saccadic latencies.

Neural Enhancement and Pre-Emptive Perception: The Genesis of Attention and the Attentional Maintenance of the Cortical Salience Map

One of the stable hypotheses in systems neuroscience is the relationship between attention and the enhancement of visual responses when an animal attends to the stimulus in its receptive field (Goldberg and Wurtz, 1972 Journal of Neurophysiology 35 560 – 574). This was first discovered in the superior colliculus of the monkey: neurons in the superficial layers of the superior colliculus responded more intensely to the onset of a stimulus during blocks of trials in which the monkey had to make a saccade to it than they did during blocks of trials in which the monkey had to continue fixating a central point and not respond to the stimulus. This enhancement has been found in many brain regions, including prefrontal cortex (Boch and Goldberg, 1987 Investigative Ophthalmology 28 Supplement, 124), V4 (Moran and Desimone, 1985 Science 229 782 – 784), and lateral intraparietal area (Colby et al, 1996 Journal of Neurophysiology 76 2841 – 2852; Colby and Goldberg, 1999 Annual Review of Neuroscience 22 319 – 349), and even V1 (Lamme et al, 2000 Vision Research 40 1507 – 1521). In these studies the assumption has been that the monkey attended to the stimulus because the stimulus evoked an enhanced response. In the experiments described here we show that for abruptly appearing stimuli, attention is not related to the initial response evoked by the stimulus, but by the activity present on the salience map in the parietal cortex when the stimulus appears. Attention to the stimulus may subsequently, by a top – down signal, sustain the map, but stimuli can as easily be suppressed by top – down features as they can be enhanced.

Feature attention evokes task-specific pattern selectivity in V4 neurons

A hallmark of visual cortical neurons is their selectivity for stimulus pattern features, such as color, orientation, or shape. In most cases this feature selectivity is hard-wired, with selectivity manifest from the beginning of the response. Here we show that when a task requires that a monkey distinguish between patterns, V4 develops a selectivity for the sought-after pattern, which it does not manifest in a task that does not require the monkey to distinguish between patterns. When a monkey looks for a target object among an array of distractors, V4 neurons become selective for the target ∼50 ms after the visual latency independent of the impending saccade direction. However, when the monkey has to only make a saccade to the spatial location of the same objects without discriminating their pattern, V4 neurons do not distinguish the search target from the distractors. This selectivity for stimulus pattern develops roughly 40 ms after the same neurons’ selectivity for basic pattern features like orientation or color. We suggest that this late-developing selectivity is related to the phenomenon of feature attention and may contribute to the mechanisms by which the brain finds the target in visual search.

Activity in V4 Reflects the Direction, But Not the Latency, of Saccades During Visual Search

We constantly make eye movements to bring objects of interest onto the fovea for more detailed processing. Activity in area V4, a prestriate visual area, is enhanced at the location corresponding to the target of an eye movement. However, the precise role of activity in V4 in relation to these saccades and the modulation of other cortical areas in the oculomotor system remains unknown. V4 could be a source of visual feature information used to select the eye movement, or alternatively, it could reflect the locus of spatial attention. To test these hypotheses, we trained monkeys on a visual search task in which they were free to move their eyes. We found that activity in area V4 reflected the direction of the upcoming saccade but did not predict the latency of the saccade in contrast to activity in the lateral intraparietal area (LIP). We suggest that the signals in V4, unlike those in LIP, are not directly involved in the generation of the saccade itself but rather are more closely linked to visual perception and attention. Although V4 and LIP have different roles in spatial attention and preparing eye movements, they likely perform complimentary processes during visual search.

Opposing views on orthogonal adaptation: a response to Clifford, Arnold, Smith, and Pianta (2003)

Clifford et al., 2003 point out that there is indeed a subtle difference between their experiment and ours (Westheimer & Gee, 2002). Because it is generally better to provide the observer with an explicit standard when measuring difference thresholds we had interposed a brief presentation of the comparison before showing the test pattern for which the orientation judgment had to be made. Clifford et al. now question whether this could undo the adapting effect of orthogonal gratings. Accordingly we modified the procedure used in our original experiments by blanking out the frames which at that time carried the comparison information; now the orthogonal adaptation was interrupted only to show the test pattern, suitably buffered by a blank screen. 1 The results for two observers are given in Table 1. With a foveal line stimulus, adaptation to a set of orthogonal stripes still does not change orientation discrimination thresholds. For the pattern duplicating Clifford et al. s, where two peripheral circular grating patches are subjected to orthogonal adaptation with similar parameters, one of our observers again had no adaptation effect. The other had a 15% reduction in threshold in the adapted compared to the non-adapted situation, just missing the 5% level of significance. This result is based on five sessions, spread over three experimental days always with properly counterbalanced sequences of adaptated/nonadapted conditions. In the original experiment with this pattern, this observer had an adapted threshold 4% higher than the one without adaptation. Thus, in this case, the presence of a comparison stimulus could have gone same way towards blocking the ability of orthogonal adaptation to improve orientation discrimination thresholds. However, this effect is not manifested by all observers and certainly not for all patterns. If it turns out that a brief presentation of a test pattern nulls out some adaptation phenomena, it is interesting to speculate on the origin of such an effect. One possibility is that the adapting pattern, although it had already been extinguished, provided some sort of an orientation framework that becomes diluted when there is an intercalated reference. Another possibility would involve transient changes in the orientation tuning of cortical neurons immediately subsequent to some specific stimuli, a phenomenon that has been demonstrated in a related context in the cat visual cortex (Yao & Dan, 2001). The challenge of Clifford et al. s original aim remains: the quest for a neural substrate for the opponency mechanism underlying the tilt illusion s reversal when the inducing pattern approaches orthogonality. The patterns used in this research all evoke both a direct and indirect tilt illusion. The predominant absence of an influence of orthogonal adaptation implies that orientation discrimination thresholds are not tightly enough coupled to the apparatus generating the mean orientation shifts to assign them a secure role as an analytical tool in this quest. * Corresponding author. Tel.: +1-510-642-4828; fax: +1-510-6436791. E-mail address: gwest@socrates.berkeley.edu (G. Westheimer). 1 This modification in the procedure is feasible only for orientation discrimination thresholds in the vertical or horizontal meridians, where a comparison with a standard can be dispensed with because observers have a strong internal representation of the reference orientation. The extension of the experiment to oblique meridians, as was done in obtaining the results in Fig. 5 of our original communication, would not be possible without providing the observer an explicit reference.