Bonnie M. Lawrence

The effects of eye and limb movements on working memory

Three experiments examined the role of eye and limb movements in the maintenance of information in spatial working memory. In Experiment 1, reflexive saccades interfered with memory span for spatial locations but did not interfere with memory span for letters. In Experiment 2, three different types of eye movements (reflexive saccades, pro-saccades, and anti-saccades) interfered with working memory to the same extent. In all three cases, spatial working memory was much more affected than verbal working memory. The results of these two experiments suggest that eye movements interfere with spatial working memory primarily by disrupting processes localised in the visuospatial sketchpad. In Experiment 3, limb movements performed while maintaining fixation produced as much interference with spatial working memory as reflexive saccades. These results suggest that the interference produced by eye movements is not the result of their visual consequences. Rather, all spatially directed movements appear to have similar effects on visuospatial working memory.

An anti-Hick's effect in monkey and human saccade reaction times

In order to execute movements to targets in the environment, we must first select a target in which to move, generally from an array of alternatives. Hicks Law states that reaction time (RT) increases as the number of response alternatives increases. Violations of this law, however, generally in the form of the absence of a relationship between response alternatives and RT have been reported in the literature. K. Kveraga, L. Boucher, and H. C. Hughes (2002), for example, found that saccades to visual targets violate Hicks Law. To examine this violation further, we measured saccade RTs in monkeys and humans and found that saccade RTs actually decreased as the number of potential target locations increased. We hypothesize that this arises because subjects must actively inhibit premature saccades, and that the required inhibition increases as the certainty of a movement to a particular location increases. With increased inhibition, saccade onset is delayed, resulting in an anti-Hicks effect.

Reach preparation enhances visual performance and appearance

We investigated the impact of the preparation of reach movements on visual perception by simultaneously quantifying both an objective measure of visual sensitivity and the subjective experience of apparent contrast. Using a two-by-two alternative forced choice task, observers compared the orientation (clockwise or counterclockwise) and the contrast (higher or lower) of a Standard Gabor and a Test Gabor, the latter of which was presented during reach preparation, at the reach target location or the opposite location. Discrimination performance was better overall at the reach target than at the opposite location. Perceived contrast increased continuously at the target relative to the opposite location during reach preparation, that is, after the onset of the cue indicating the reach target. The finding that performance and appearance do not evolve in parallel during reach preparation points to a distinction with saccade preparation, for which we have shown previously there is a parallel temporal evolution of performance and appearance. Yet akin to saccade preparation, this study reveals that overall reach preparation enhances both visual performance and appearance.

General Slowing of Lexical and Nonlexical Information Processing in Dementia of the Alzheimer Type

Individuals with very mild and mild dementia of the Alzheimer type (DAT) and age-matched controls performed three lexical and three nonlexical speeded information-processing tasks. The DAT group was slower than the control group on all six tasks. As predicted by general slowing (Nebes & Brady, 1992), most main effects of task condition were accompanied by Group × Condition interactions. That is, as task complexity increased, the response times (RTs) of the DAT group increased more than the RTs of the control group. Multitask regression analyses confirmed the existence of general slowing in DAT, such that the DAT group took approximately 1.8 times as long to process information as the controls on all six tasks. Importantly, lexical and nonlexical processing speed were equivalently affected by DAT. This pattern was observed both in very mild and mild DAT, although the degree of general slowing increased with the severity of the dementia.

An anti-Hick's effect for exogenous, but not endogenous, saccadic eye movements.

Previously, we have shown that the reaction times (RTs) of exogenously generated saccadic eye movements decrease with an increase in the number of response alternatives (Lawrence et al. in J Vis 8(26):1–7, 2008; Lawrence and Gardella in Exp Brain Res 195(3):413–418, 2009). Because this pattern of RTs is in the direction opposite that predicted by Hick (Q J Exp Psychol 4:11–26, 1952), we termed the effect an “anti-Hick’s” effect. In the present study, we examined whether this effect characterizes saccades in general, or only those saccades that are exogenously generated. An anti-Hick’s effect was found for exogenous, but not for endogenous, saccades. These results demonstrate a clear dissociation between exogenously and endogenously generated saccades and place an important constraint on the anti-Hick’s effect.

Saccades and reaches, behaving differently

Previously, we have shown, both in humans and monkeys, that the latencies of exogenously generated saccades decrease with an increase in the number of response alternatives (Lawrence et al. in J Vis 8:26, 1–7, 2008). Because this pattern of latencies was in the direction opposite that predicted by Hick (Q J Exp Psychol 4:11–26, 1952), we termed the effect an “anti-Hick’s” effect. In contrast, previous research has shown that reach latencies increase with an increase in response alternatives (e.g., Wright et al. in Exp Brain Res 179:475–496, 2007). Given that there are known interactions between the saccade and reach systems, we examined whether the direction of the relationship between latencies and response alternatives differed when saccades and reaches are concomitantly executed. Interestingly, we found that the pattern of latencies nevertheless persisted in a visually guided saccade and reach task. These results place an important constraint on the anti-Hick’s effect, suggesting not only that the effect is localized within the saccade system, but also that it is localized in the saccade system at a level in which saccade and reach signals do not interact.

Don't go there.

Response inhibition, or impulse control, is critical for normal cognitive function. In this issue of Neuron, Hasegawa and colleagues use a spatial nonmatch-to-sample task to reveal neurons in and around the frontal eye fields that encode where an animal should not look.

The Responses of Visual Neurons in the Frontal Eye Field Are Biased for Saccades

Previous research suggests that visually responsive neurons in the frontal eye field (FEF) respond to visual targets even when they are not the goal of a saccadic eye movement. These results raise the possibility that these neurons respond to visual targets independent of the effector that is to be used to acquire the target locations. In the present study, we examined whether a plan to execute a saccade or a reach to a visual target influenced the response to and the representation of targets in the FEF. We recorded single unit responses to the onset of the target, during the delay period, and around the time of the movement, on interleaved saccade and reach trials of a delayed-response task. We found that the responses of approximately equal percentages of visual, visuomovement, and movement neurons (50%, 58%, and 58%, respectively) were greater on saccade trials than on reach trials in at least one interval of the delayed-response task. Converse biases, in favor of reaches, were much less frequent (13%, 10%, and 19%, in visual, visuomovement, and movement neurons respectively). Thus, although visual neurons may not be directly involved in triggering saccadic eye movements, they are nonetheless highly saccade-biased, with percentages comparable to neurons that are directly involved in triggering saccadic eye movements.

Manipulations of the relationship between response alternatives and exogenous saccade latencies

The relationship between the latencies of saccadic eye movements and the number of response alternatives is complex. Previously, we have found a decrease in exogenous saccade latencies with an increase in the number of response alternatives (i.e., an anti-Hick’s effect). In the present study, we examined the effect of bottom up and top down influences on the effect. In Experiment 1, we found that the anti-Hick’s effect is dependent upon the number, as opposed to the configuration, of response alternatives, suggesting that the effect is not a purely bottom up phenomenon. In Experiment 2, we examined whether top down processes, such as expectancy, influence the magnitude and/or direction of the relationship between response alternatives and reaction time. We found that interleaving, as opposed to blocking, the number of alternatives negated the anti-Hick’s effect. Taken together, these results suggest that the anti-Hick’s effect arises not only from stimulus-based perceptual signals but also from higher-order control signals.

Region-Specific Summation Patterns Inform the Role of Cortical Areas in Selecting Motor Plans

Given an instruction regarding which effector to move and what location to move to, simply adding the effector and spatial signals together will not lead to movement selection. For this, a nonlinearity is required. Thresholds, for example, can be used to select a particular response and reject others. Here we consider another useful nonlinearity, a supralinear multiplicative interaction. To help select a motor plan, spatial and effector signals could multiply and thereby amplify each other. Such an amplification could constitute one step within a distributed network involved in response selection, effectively boosting one response while suppressing others. We therefore asked whether effector and spatial signals sum supralinearly for planning eye versus arm movements from the parietal reach region (PRR), the lateral intraparietal area (LIP), the frontal eye field (FEF), and a portion of area 5 (A5) lying just anterior to PRR. Unlike LIP neurons, PRR, FEF, and, to a lesser extent, A5 neurons show a supralinear interaction. Our results suggest that selecting visually guided eye versus arm movements is likely to be mediated by PRR and FEF but not LIP.