Effects of spatial separation between stimuli in whole report from brief visual displays

Abstract

Direct measurements of the effects of spatial separation between stimuli in whole report from brief visual displays are reported. The stimuli were presented on the periphery of an imaginary circle centered on fixation. In Experiment 1, each display showed two capital letters (letter height 1.3º, width 0.9º, eccentricity 5.5º). The proportion of correctly reported letters was a strictly increasing, decelerating function of the spatial separation between the letters for center-to-center separations ranging from less than 2º to more than 10º of visual angle. Experiment 2 yielded similar results with triples of letters. Experiment 3 showed that accuracy increased with spatial separation for report of two short words, and Experiment 4 showed the same result for words presented upside down. The results are explained by a model of lateral masking (crowding) based on competitive interactions within receptive fields of cortical neurons.

Critical resolution: A superior measure of crowding

“…The central idea of such models is that stimuli compete for neuronal representation when multiple stimuli fall into the receptive field of the same neuron (Moran & Desimone, 1985). This approach has previously been used to derive a quantitative explanation of crowding data (Kyllingsbaek, Valla, Vanrie, & Bundesen, 2007) based on the idea that crowding results from such competitive interactions between stimuli. The extent of competition for processing resources depends on how many neurons have both stimuli within their receptive fields.…”

The dynamics of buffered and triggered selection from rapid serial visual presentation (RSVP) streams.

“…Although crowding is unlikely to have occurred in our experiments, stimuli with larger spacings may sometimes yield interference effects (Kyllingsbaek et al, 2007). To investigate whether this is the reason for the effect of number of streams in Experiments 1 and 2, we hold the level of potential interference between stimulus representations constant in Experiment 3 by always presenting the same number of streams on each trial.…”

Don't words come easy? A psychophysical exploration of word superiority

“…Another possible explanation of the reversed effect in the whole report experiment is that stimuli were presented outside the central visual field (at 4.6° from fixation). Although previous work indicates that there is little crowding between stimuli at this eccentricity Kyllingsbæk et al, 2007), “within stimulus crowding” (i.e., lateral masking) may have affected the processing of words in this condition. Jordan and Patching (2004) have shown that the word-letter phenomenon can be reversed when stimuli are presented in lateralized displays, which resembles the effect we find in Experiment 3.…”

“…Stimuli were presented for one of six exposure durations (30–200 ms, randomly intermixed), and followed by either six pattern masks (500 ms), or a blank screen prolonging the effective exposure duration by a visual afterimage. Stimuli were shown at six locations on the circumference of an imaginary circle with a radius of 4.6° of visual angle centered on fixation(given this radius and the size of the words and letters used crowding effects between stimuli are minimal, see Kyllingsbæk et al, 2007). Again the instruction was to make unspeeded reports of the items which the subject was “fairly certain” of having seen, and responses were recorded by the experimenter.…”