Temporal summation in human vision: simple reaction time measurements

Effect of the retinal size of a peripheral cue on attentional orienting in two- and three-dimensional worlds

It has been documented that due to limited attentional resources, the size of the attentional focus is inversely correlated with processing efficiency. Moreover, by adopting a variety of two-dimensional size illusions induced by pictorial depth cues (e.g., the Ponzo illusion), previous studies have revealed that the perceived, rather than the retinal, size of an object determines its detection. It remains unclear, however, whether and how the retinal versus perceived size of a cue influences the process of attentional orienting to subsequent targets, and whether the corresponding influencing processes differ between two-dimensional (2-D) and three-dimensional (3-D) space. In the present study, we incorporated the dot probe paradigm with either a 2-D Ponzo illusion, induced by pictorial depth cues, or a virtual 3-D world in which the Ponzo illusion turned into visual reality. By varying the retinal size of the cue while keeping its perceived size constant (Exp. 1), we found that a cue with smaller retinal size significantly facilitated attentional orienting as compared to a cue with larger retinal size, and that the effects were comparable between 2-D and 3-D displays. Furthermore, when the pictorial background was removed and the cue display was positioned in either the farther or the closer depth plane (Exp. 2), or when both the depth and the background were removed (Exp. 3), the retinal size, rather than the depth, of the cue still affected attentional orienting. Taken together, our results suggest that the retinal size of a cue plays the crucial role in the visuospatial orienting of attention in both 2-D and 3-D.

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%, 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.

Mechanisms of human motion perception: combining evidence from evoked potentials, behavioural performance and computational modelling

Based on single cell recordings in monkey, it has been suggested that neural activity can be related directly to psychophysically measured threshold behaviour. Here, we investigated in humans whether evoked potentials correlate with behavioural measurements like discrimination thresholds and reaction time. Subjects were asked to report the perceived direction of object motion stimuli which contained variable amounts of coherent motion. Simultaneously, we recorded evoked potentials with a multielectrode array, or measured the reaction time. We show here that motion coherence had a strong influence on both amplitude and latency of the evoked potential. Stronger motion signals evoked stronger and faster cortical responses. The latency reduction of the motion onset response with increasing coherence correlated very well with the concurrent decrease in reaction time. Taken together, these results suggest that temporal integration is an important step in analysing motion signals to generate a reliable behavioural response. We stimulated a two-dimensional array of correlation-type motion detectors with the same motion sequences, and analysed the distribution of local motion signals according to signal detection theory. Performance resembled that of human subjects when the decision strategy was optimized so as to exclude small signals and, in particular, when the ideal observer had some knowledge about a region of interest in which the object was to be expected.

Characteristics of anisometropic suppression: simple reaction time measurements

The characteristics of artificially induced anisometropic suppression were investigated in observers with normal and abnormal binocular vision (anisometropic amblyopia) by using a simple reaction time paradigm. Reaction time was measured as a function of stimulus intensity for various stimulus durations. For all conditions, the reaction time increased as stimulus intensity decreased toward threshold. We found that traditional techniques for modeling this trend were inadequate, so we developed a simple visuogram method for comparing these functions. Using this technique, reaction time versus intensity functions are shown to be shape-invariant for all conditions examined. This means that, although reaction times are longer during induced anisometropic suppression or in anisometropic amblyopia, they are the same if contrast is normalized to equate threshold. The shape-invariant nature of these functions is also consistent with the notion that a single mechanism mediates detection under these conditions. Temporal summation was investigated at both threshold (method of limits) and suprathreshold (criterion reaction time) levels. Again, because of shape invariance, the suprathreshold results mirror the threshold results. The critical duration (the duration at the intersection of the complete summation and zero summation regions) is not affected by any of the conditions. However, the critical intensity (the intensity for the zero summation region) is higher for the amblyopic eyes, as compared with the normal or nonamblyopic eyes. Induced anisometropic suppression always increases the critical intensity, with a smaller increase occurring for the amblyopic eyes. This suggests that amblyopic eyes do not have a need for strong suppression.

Simple reaction time and perception of temporal order: dissociations and hypotheses

Simple motor reaction time and judgment of temporal order are commonly recognized as two methods for estimation of perceptual latency. Unfortunately, the results obtained by the methods under the same conditions do not agree. We review hypotheses attempting to explain the disagreement. Although some of these seem to be promising, no one at present could be fully accepted.

Analysis of simple reaction time to a sinusoidal grating by means of a linear filter model of the detection process

Simple reaction time (RT) to a sinusoidal grating was analyzed in terms of a linear filter model of the detection process. First, RT contrast functions were determined over a wide range of spatial frequencies and retinal illuminances. Second, calculating the time course of the linear filter's response, theoretical visual latency contrast functions were derived for the same conditions of spatial frequency and retinal illuminance as those in the RT measurements. Comparison of the two functions showed that the contrast dependence of the RT functions was much larger than that of the visual latency functions. The discrepancy between the two functions was satisfactorily described as a power function of the slope of the filter's response at threshold level. On the basis of these results, we propose a model of the RT process. According to the model, the RT process is mediated by a cascade that consists of a level detector, which includes a linear filter followed by a threshold device, and a differentiator of the filter's response.

A theoretical model for perceived intensity in human taste and smell. II. Temporal integration and reaction times

The theoretical model for perceived intensity in human taste and smell published previously was extended by incorporating the concept of signal detectability and temporal integration phenomena at low and high stimulus levels. The processes involved in human taste and smell perception are divided into three stages: a) An "internal meter" registers outside signals. Adaptation occurs at this stage. b) The meter is read for two possible purposes: to detect a weak signal or to estimate the signal's intensity. Both tasks require the smoothing of noise, which is accomplished by integration of the meter over time. c) The internal estimate is expressed through the use of a scaling method.

Spatial summation in human vision: simple reaction time measurements

The reaction-time technique was applied to examine spatial summation or area-intensity reciprocity at suprathreshold levels in the fovea. A family of reaction time vs luminance curves was measured in two experiments, and the luminance required to produce a criterion reaction time was computed from these curves to estimate the extent of summation. The first experiment showed that the upper level of spatial summation, the Ricco area, defined for constant reaction time increases with decreasing luminance level and that the upper limit of temporal summation is independent of the change in target size. The second indicated that the Ricco area decreases with increasing pulse duration up to 20-30 ms and then remains constant.