Academic journal article Perception and Psychophysics

Does Attention Alter Appearance?

Academic journal article Perception and Psychophysics

Does Attention Alter Appearance?

Article excerpt

Abrupt onsets in the visual field can change the appearance of subsequent stimuli, according to one interpretation, by engaging an attentional mechanism that increases effective stimulus contrast However, abrupt onsets can also engage capacity-unlimited and thus attention-independent sensory mechanisms. We conducted a series of experiments to differentiate the sensory and attentional accounts. Observers compared the contrasts of uncued low-contrast peripheral targets with simultaneous targets cued by one of three cue types with different sensory attributes: white or black peripheral abrupt onsets and central gaze direction cues devoid of sensory activity near the target locations. Each cue facilitated the perception of perithreshold targets; however, the white abrupt onsets increased the perceived contrast of suprathreshold targets, whereas the black abrupt onsets tended to reduce the perceived contrast, and the gaze direction cues had no significant effect The effectiveness of the gaze direction cues in automatically orienting attention was demonstrated in a control experiment in which they consistently speeded response times. The results suggest that sensory interaction, and not attention, is responsible for changes in appearance.

(ProQuest Information and Learning: ... denotes formulae omitted.)

A common feature of our nervous system is that the property of intensity-for example, the loudness of a tone, the luminance contrast of a light (its brightness relative to the background), or the strength of a muscular contraction-is encoded by the discharge frequency of neurons. Attention to a stimulus increases that stimulus's evoked firing rate, and, in the early visual system, attention ostensibly functions to increase the effective contrast of a stimulus, operating through what has become known as the contrast gain model of attention. For example, in neurons in areas V4 and MT of the macaque visual cortex, attention directed to a stimulus increases firing rates similar to an increase in the stimulus contrast (Martínez-Trujillo & Treue, 2002; Reynolds, Pasternak, & Desimone, 2000). In macaque V4, attention has an effect on orientation tuning curves of single neurons similar to that of contrast increments in the cat visual cortex, multiplicatively increasing the amplitude without changing the tuning width (McAdams & Maunsell, 1999; Sclar & Freeman, 1982; Skottun, Bradley, Sclar, Ohzawa, & Freeman, 1987).

The perceptual consequences of this apparent interchangeability between attention and contrast (the contrast gain model) in the early visual cortex are unclear. The independence of attention and the intensity of perception have been important questions in experimental psychology since the late nineteenth century (Newhall, 1921 ; Pillsbury, 1908; Titchener, 1908), but several methodological difficulties have hindered progress. First, conscious perception is difficult to study physiologically because its neural correlates remain unclear (e.g., Helekar, 1999; Markowitsch, 1995; Pollen, 1999; Rees, Kreiman, & Koch, 2002). Subjective comparisons between test and reference stimuli are the best we can do to study perception as a dependent variable. second, to modulate attention as the independent variable within this paradigm, attention must be differentially allocated between the two stimuli. The difficulty of this was noted by Pillsbury (1908):

It is impossible directly to compare an object attended to with one not attended to. There is an unavoidable impulse to attend to both before the judgment is made, and any results that should claim to be accurate on this point would be open to grave suspicion, (p. 4)

The problem has persisted because many modern methods of orienting attention, such as central arrows pointing toward locations in peripheral visual space (Posner, 1980), require volitional compliance; that is, observers must voluntarily shift their attention, and the cues may be easily ignored (Jonides, 1981). …

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