Voluntary attention increases perceived spatial frequency

Abstract

Voluntary covert attention selects relevant sensory information for prioritized processing. The behavioral and neural consequences of such selection have been extensively documented, but its phenomenology has received little empirical investigation. Involuntary attention increases perceived spatial frequency (Gobell & Carrasco, 2005), but involuntary attention can differ from voluntary attention in its effects on performance in tasks mediated by spatial resolution (Yeshurun, Montagna, & Carrasco, 2008). Therefore, we ask whether voluntary attention affects the subjective appearance of spatial frequency--a fundamental dimension of visual perception underlying spatial resolution. We used a demanding rapid serial visual presentation task to direct voluntary attention and measured perceived spatial frequency at the attended and unattended locations. Attention increased the perceived spatial frequency of suprathreshold stimuli and also improved performance on a concurrent orientation discrimination task. In the control experiment, we ruled out response bias as an alternative account by using a lengthened interstimulus interval, which allows observers to disengage attention from the cued location. In contrast to the main experiment, the observers showed neither increased perceived spatial frequency nor improved orientation discrimination at the attended location. Thus, this study establishes that voluntary attention increases perceived spatial frequency. This phenomenological consequence links behavioral and neurophysiological studies on the effects of attention.

Figure 1

Schematic depiction of the trial sequence. For ease of illustration, the cues are depicted as a brightening of all or part of the horizontal bar of the fixation cross, whereas in the experiment, the cues were presented as a thickening of all or part of this bar. Also for illustrative purposes, the tilt of the Gabors is exaggerated, and only four rapid serial visual presentation (RSVP) frames are shown. The inset depicts the response scheme: If an X was present in the RSVP stream, the participants pressed the space bar; otherwise, they were asked to report the orientation of the Gabor stimulus with the higher spatial frequency, using the right hand if it was the Gabor on the right and the left hand if it was the Gabor on the left. For either hand, one of two possible keys was pressed to indicate whether the Gabor was tilted to the left (“\”) or to the right (“/”).

Figure 2

Rapid serial visual presentation (RSVP) performance in the neutral- and attentional-cue conditions for the (A) main and (B) control experiments. RSVP performance is reported in d′ units. Error bars are within-subjects standard errors of the means, calculated using the method of Loftus and Masson (1994). ISI, interstimulus interval.

Figure 3

Results for the contrast judgments. The top row presents psychometric functions for the (A) main and (B) control experiments. The graphs show the proportions of trials on which the observers chose the test stimulus to be of higher spatial frequency than the standard stimulus, as a function of the physical spatial frequency of the test stimulus. The middle row presents the normalized values of the point of subjective equality (PSE) for the test stimulus for each of the three cue types in the (C) main and (D) control experiments. Error bars are standard errors of the means, calculated using the method of Loftus and Masson (1994). The bottom row presents scatterplots of individual observers’ normalized PSEs in the (E) main and (F) control experiments; each observer’s normalized PSEs for test stimuli in the test-cued and standard-cued conditions are plotted as a function of that observer’s PSE for the test stimuli in the neutral-cue condition. ISI, interstimulus interval. *p < .01.

Figure 4

Orientation discrimination performance (accuracy) for trials on which the observers chose the standard stimulus. Performance in the (A) main and (B) control experiments is graphed as a function of cuing condition: cued (i.e., standard cued), uncued (i.e., test cued), or neutral (i.e., neither stimulus cued). Error bars represent standard errors of the means, calculated using the method of Loftus and Masson (1994). The percentage of trials on which the observers selected the standard stimulus in each condition is indicated. ISI, interstimulus interval. *p < .05.

Figure 5

Illustration of how attention might produce a change in apparent spatial frequency via a shift in sensitivity to higher spatial frequencies. The left panel schematically illustrates the baseline sensitivity of different channels, and the right panel illustrates increased sensitivity for the channels of higher spatial frequency with attention (illustrated in light orange). The vertical bar illustrates a stimulus of a particular frequency, and the horizontal lines indicate the response of three illustrative channels.