Positive afterimages from a filled-in representation

1. Positive afterimages from a filled-in representation UCSD UCSD fixation LJ JJ AH DB TM 3 Alex O. Holcombe Don MacLeod and S. Tanner Mitten Cardiff University University of California, San Diego General Explanation The afterimages cannot be explained by persisting retinal activity. Instead, they seem to represent the persistence of a cortical representation in the absence of contrary information. When the filled-in area was replaced with grey, the insensitive scotoma of the retina was unable to signal the change. The visual system assumed that nothing had changed at the scotoma, yielding a positive afterimage of the filled-in representation. The afterimage then persisted until the retinally stabilized scotoma was again filled by its surround. Complication The afterimages were not as bright or as dark as the previously filled-in representations. Perhaps fading commences immediately upon presentation of the grey field, diluting the color of the afterimage. Summary Positive afterimages are usually explained in terms of continuing retinal activity. However, our results suggest that positive afterimages instead reflect the persistence of a cortical representation in the absence of evidence that the world has changed. Test display (afterimage is perceived) region of scotoma (and afterimage) Bleaching flash After dark adaptation, a flash creates a temporary scotoma- the area’s rods are insensitive for ~10 min Brightness reported by referring to swatches 1 2 fixation 3.5 The display is switched to the test, and observers see an afterimage filling the scotoma area. They report its brightness by referring to the reference swatches. In half of trials they report the brightness of the top end of the afterimage, in half the bottom end. 4 2 Reinterpreting previous work To our knowledge, other than in MacLeod (unpublished dissertation, 1974), positive afterimages have always been attributed to continuing activity evoked by a very bright inducing stimulus. Instead, our results suggest that the common experience of perceiving a brief afterimage of a bright light occurs when the light makes the retina locally insensitive to what comes after. One then continues to perceive the light until its representation fills in with the new surround. Observer dons bib, hat, goggles, and earflaps. The goggles’ filters cut the light to scotopic (below cone threshold) levels, and are used because the rod scotoma is long-lasting. The flash (red outlined area) was ~107,300 scotopic lux and lasted ~170 ms, bleaching ~73% of the rod pigment Results Filling-in displays White Black Additional Supporting Observation Perceived brightness of afterimage white black black bit of bipartite white bit of bipartite Filling-in display region of scotoma Filling-in display Test display Bipartite • The afterimage resulting from the white filling-in display was bright, although not as bright as the original. • The afterimage from the black display was dark, although not as dark as the original. • For the bipartite, the afterimage appeared bright where white had filled in and dark where black had filled in. After the bleaching flash and fixation on the left bullseye of the stimulus above, we saccade to the test display bullseye and see a white rectangular protrusion. No edge is seen between the two halves of the afterimage. This implies that the fading of the filled-in representation of the previous experiment does not occur here. hmmm… Before we elicit the critical afterimage, observer views one of these displays- at first the scotoma region appears different from its surround, but after a few seconds filling in occurs. In the bipartite display, this results in the edge completing as a fuzzy border between black and white regions of the scotoma. Percept