Hamilton Haddad

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 subjects 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 observers 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.

Ilusões: o olho mágico da percepção

Perception is the active construction of a neural state that correlates with biologically relevant elements present in the environment. This correlation, far from affording a one-to-one mapping, nonetheless guides our actions towards adaptive behaviors, thus being forged under evolutionary constraints. Since the construction of a percept is an intrinsically ambiguous process, perceptual discrepancies can arise from identical stimulation patterns. The recognition of these discrepancies is termed illusion, which originates, however, from the same physiological mechanisms that ordinarily lead to standard perception. Emanating from different sources, such as optical, sensory and cognitive factors, visual illusions are useful tools in accessing the physiological basis of perceptual processes and their interaction with motor planning and execution. Here we examine the biological roots of visual illusions and their interplay with some neurobiological, philosophical and esthetical issues.

Effects of intensity and positional predictability of a visual stimulus on simple reaction time.

The influence of visual stimuli intensity on manual reaction time (RT) was investigated under two different attentional settings: high (Experiment 1) and low (Experiment 2) stimulus location predictability. These two experiments were also run under both binocular and monocular viewing conditions. We observed that RT decreased as stimulus intensity increased. It also decreased as the viewing condition was changed from monocular to binocular as well as the location predictability shifted from low to high. A significant interaction was found between stimulus intensity and viewing condition, but no interaction was observed between neither of these factors and location predictability. These findings support the idea that the stimulus intensity effect arises from purely sensory, pre-attentive mechanisms rather than deriving from more efficient attentional capture.

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 (4.8 degrees and 9.6 degrees ). 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 (2.5 degrees, 7.3 degrees and 12.1 degrees ) 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.

Teaching diffusion with a coin

DIFFUSION is a key subject in physiology. By means of diffusional processes, substances can be efficiently transferred at short distances. For that reason, it is a very important mechanism of exchange between the cells comprising a plant or an animal and the surrounding compartments with which they interact as well as between the compartments themselves (such as the interstitial and capillary compartments, for example). However, diffusion is too a slow process when matter has to be transported along the large distances found in the bodies of multicellular organisms. Consequently, as organisms became larger, diffusion was no longer capable of providing for all their needs, and convective transport was made necessary. Explaining to undergraduate students why diffusion works so badly at larger distances while at the same time being a process of paramount relevance at the cellular level is not a trouble-free task (4). Although students easily grasp the notion that random processes are capable of giving rise to emergent behavior, such as the net directional movement of molecules, they frequently confound a macroscopic ballistic view of movement with a microscopic diffusive one. It appears to arise from a deep-seated misconception about random processes (2). In the present article, we describe an inexpensive and simple way to make students intuitively experience the probabilistic nature and nonorientated motion of diffusing particles. This understanding allows students to realize why diffusion works so well over short distances and becomes increasingly and rapidly less effective as the distances involved become progressively larger. We believe that this activity is in agreement with the urge for the adoption of teaching approaches that more actively involve the student in the learning process, focus on problem solving, and lead

The Time of Perception and the Other Way Around

The world we perceive is delayed in relation to its flowing content, as well as the outcome of our actions on the world in relation to the moment we decide to act. This mosaic of different latencies permeating both perception and action has to be taken into account critically in order for us to cope with the temporal challenges constantly imposed by the environment. Fundamental notions, such as the sense of agency and causality, depend on the temporal relationship of events occurring in well-defined windows of time. Here, we offer a broad, yet abridged, historical view of some thought-provoking issues concerning the time of perception and action. From the pioneering work of Wundt, Titchener, and Libet to recent findings and ideas related to the employment of visual illusions as psychophysical probes (such as the flash-lag effect), we have tried to expose some problems inherent to the act of measuring the time of both perception and action, and devise possible solutions as well.

Componentes sensoriais e atencionais do tempo de reação: efeitos do tamanho, excentricidade e previsibilidade de estímulos visuais

A percepcao visual depende do arcabouco sensorial e do processamento atencional. Este trabalho estudou os efeitos do tamanho, excentricidade e previsibilidade de um estimulo visual sobre tempo de reacao manual (TR). No Experimento I (n=8), um alvo foi apresentado aleatoriamente em quatro excentricidades diferentes, possuindo tres possiveis tamanhos. O Experimento II (n=12) apresentava configuracao similar, porem uma pista indicava o quadrante de apresentacao do alvo. Os resultados mostraram um aumento do TR em funcao da excentricidade do alvo, alem de uma diminuicao do TR com o aumento do tamanho do alvo e indicacao correta da pista. Uma analise das interacoes dos fatores estudados sugere uma superposicao de mecanismos atencionais e sensoriais em pelo menos um estagio comum do processamento visual.