The modulation of simple reaction time by the spatial probability of a visual stimulus

Simple reaction time (SRT) in response to visual stimuli can be influenced by many stimulus features. The speed and accuracy with which observers respond to a visual stimulus may be improved by prior knowledge about the stimulus location, which can be obtained by manipulating the spatial probability of the stimulus. However, when higher spatial probability is achieved by holding constant the stimulus location throughout successive trials, the resulting improvement in performance can also be due to local sensory facilitation caused by the recurrent spatial location of a visual target (position priming). The main objective of the present investigation was to quantitatively evaluate the modulation of SRT by the spatial probability structure of a visual stimulus. In two experiments the volunteers had to respond as quickly as possible to the visual target presented on a computer screen by pressing an optic key with the index finger of the dominant hand. Experiment 1 (N = 14) investigated how SRT changed as a function of both the different levels of spatial probability and the subject's explicit knowledge about the precise probability structure of visual stimulation. We found a gradual decrease in SRT with increasing spatial probability of a visual target regardless of the observer's previous knowledge concerning the spatial probability of the stimulus. Error rates, below 2 percent, were independent of the spatial probability structure of the visual stimulus, suggesting the absence of a speed-accuracy trade-off. Experiment 2 (N = 12) examined whether changes in SRT in response to a spatially recurrent visual target might be accounted for simply by sensory and temporally local facilitation. The findings indicated that the decrease in SRT brought about by a spatially recurrent target was associated with its spatial predictability, and could not be accounted for solely in terms of sensory priming

Modulation of the perception of temporal order by attentional and pre-attentional factors

When two stimuli are presented simultaneously to an observer, the perceived temporal order does not always correspond to the actual one. In three experiments we examined how the location and spatial predictability of visual stimuli modulate the perception of temporal order. Thirty-two participants had to report the temporal order of appearance of two visual stimuli. In Experiment 1, both stimuli were presented at the same eccentricity and no perceptual asynchrony between them was found. In Experiment 2, one stimulus was presented close to the fixation point and the other, peripheral, stimulus was presented in separate blocks in two eccentricities (4.8º and 9.6º). We found that the peripheral stimulus was perceived to be delayed in relation to the central one, with no significant difference between the delays obtained in the two eccentricities. In Experiment 3, using three eccentricities (2.5º, 7.3º and 12.1º) for the presentation of the peripheral stimulus, we compared a condition in which its location was highly predictable with two other conditions in which its location was progressively less predictable. Here, the perception of the peripheral stimulus was also delayed in relation to the central one, with this delay depending on both the eccentricity and predictability of the stimulus. We argue that attentional deployment, manipulated by the spatial predictability of the stimulus, seems to play an important role in the temporal order perception of visual stimuli. Yet, under whichever condition of spatial predictability, basic sensory and attentional processes are unavoidably entangled and both factors must concur to the perception of temporal order

Interaction between facilitatory and inhibitory effects due to voluntary and automatic covert orienting of attention

Covert orienting of attention to one spatial location improves the processing of signals occurring at this location at the expenses of the processing of signals occurring at other spatial positions. According to the premotor theory of visual attention, the voluntary orienting of attention to a peripheral position corresponds to the programming of a saccadic eye movement towards this position. A similar mechanism has been proposed to explain the inhibitory effects elicited by a non-informative peripheral cue. This review discusses some neural mechanisms involved in the facilitatory and inhibitory effects due to covert orienting of attention.

Onset and offset inhibitions: effect of increase and decrease of cue luminance on manual reaction time to a visual target

Simple reaction time (RT) to a peripheral visual target is shortened when a non-informative cue is flashed at the target location 100-150 ms before target onset (early facilitation). With longer intervals, RT to targets appearing at cue hemifield is lengthened (inhibition of return). In the present study, we investigated these effects inverting the stimulus contrast in relation to background to see how these effects are related to the onset and/or to the offset of a cue darker or brighter than background. Ten subjects were asked not to respond to a non-informative cue (S1) appearing on a computer screen 6§ to the right or to the left of the center of a fixation cross (FP), but to respond, by pressing a microswitch, to a target (S2) occurring at 4§ from the FP in the same hemifield as S1 or in the opposite hemifield. There were two different types of sessions. In one, S1 and S2 were bright against a dark background and in the other, S1 and S2 were dark against a bright background. In each session there were two types of trials. In OFF trials, each trial began with the presentation of FP. Five hundred ms later, S1 appeared and remained on for 700 ms. S2 appeared 100 or 800 ms after the offset of S1. In ON trials, S1 onset occurred 1200 ms after the beginning of the trial and remained on until the end of trial. S2 appeared 100 or 800 ms after S1 onset. When S2 onset followed S1 onset by 100 ms, RT to S2 occuring in the same hemifield did not differ from RT when S1 and S2 were in opposite hemifields. In contrast, after an S1-S2 interval of 800 ms, S1 onset elicited an inhibition of its hemifield. This inhibition was similar to that observed 100 ms or 800 ms after S1 offset. The same results were obtained if the cue and target were brighter or darker than the background, showing that the ONSET and OFFSET inhibitions are related not to cue luminance increase and decrease, respectively, but to the appearance and the disappearance of a salient stimulus in a homogeneous background. Moreover, these results suggest that ON and OFF channels of the visual system have similar effects on the orienting of attention

Spatial distribution of the inhibition elicited by the offset of a visual cue on manual reaction time to a visual target

Simple reaction time (RT) to a peripheral visual target (S2) is shortened when a non-informative cue (S1) is flashed at the S2 location 100-150 ms before target onset (early facilitation). Afterwards, RTs to targets appearing at the S1 location are lengthened (inhbition of return). In the present investigation we studied the spatial distribution of the inhibition elicited by the offset of S1. Twelve subjects were asked not to respond to S1 which appeared on a horizontal meridian located 5.5§ above the fixation point (FP), but to respond, by pressing a key, to a target (S2) occurring at 5.5§ to the left or to the right. S1 could appear at one of 9 locations along this meridian (5.5, 3.5, 1.5, and 0.5§ to the left, 0.0 and 0.5, 1.5, 3.5, and 5.5§ to the right) and S2 occurred only at the most eccentric positions. Each trial began with the presentation of FP. Five-hundred ms later, S1 appeared and remained on for 700 ms. One hundred or 800 ms after S1 offset, S2 appeared in the same or in the opposite hemifield. We found that the offset of S1 elicits an inhibition (OFF-inhibition) which has the following features: a) it is maximal at cue's position; b) it spreads to other positions in the cued hemifield, and c) it decreases when the time interval between S1 offset and S2 onset increases from 100 to 800 ms