Visual Mental Imagery and Visual Perception: Structural Equivalence Revealed by Scanning Processes

Article excerpt

The research reported in the present article investigates whether information is represented the same way in both visual mental imagery and the early phases of visual perception. In Experiment 1, the same participants scanned over patterns of dots in a mental image (with images based on a just-seen pattern), during perception, and in an iconic image. The time to scan increasing distances increased at comparable rates in the three tasks. However, in Experiment 2, when mental images were created from information stored in long-term memory, participants scanned more slowly in the mental image condition. Nevertheless, the rates of scanning in the perceptual tasks were highly correlated with the rates of scanning in the imagery tasks in both experiments. The results provide evidence that mental images and perceived stimuli are represented similarly and can be processed in the same way.

Mental images arise from perceptual representations that are created from stored information-not from information currently being registered by the senses. According to one theory, such images arise "when a representation of the type created during the initial phases of perception is present but the stimulus is not actually perceived" (Kosslyn, Thompson, & Ganis, 2006, p. 4). And, in fact, many previous studies have documented functional similarities between visual mental imagery and perception; these studies have relied on comparing behavior during imagery and perception tasks (e.g., Denis, 1991; Finke, 1985; Kosslyn, 1980; Paivio, 1986; Perky, 1910; Segal & Fusella, 1970; Shepard & Cooper, 1982), evaluating the effects of brain damage on how well patients perform the two types of tasks (e.g., Basso, Bisiach, & Luzzatti, 1980; Bisiach & Luzzatti, 1978; Farah, 1984; Farah, Levine, & Calvanio, 1988), and comparing recordings of activation in the brain while participants engage in the two types of tasks (e.g., Ganis, Thompson, & Kosslyn, 2004; Ghaëm et al., 1997; Ishai, Ungerleider, Martin, & Haxby, 2000; Kosslyn & Thompson, 2003; Kosslyn, Thompson, & Alpert, 1997; Mellet et al., 2000).

However, some findings have challenged the claim that visual mental imagery and visual perception rely on common underlying representations. For example, Behrmann, Winocur, and Moscovitch (1992) reported that a braindamaged patient with a left homonymous hemianopia and a possible bilateral thinning of the occipital lobes (as revealed by positron emission tomography and magnetic resonance imaging) had disrupted object recognition but intact visual mental imagery. Moreover, Guariglia, Padovani, Pantano, and Pizzamiglio (1993) reported the reverse dissociation in a patient who had a large lesion involving the right frontal lobe and the anterior temporal lobe. Denis and Kosslyn (1999), Ganis et al. (2004), Kosslyn (1994), and others (see Craver-Lemley & Reeves, 1987, 1992) have suggested that this double dissociation between visual imagery and visual perception arises because forming an image relies on top-down processes that are not always necessary in perception, whereas visual perception relies on bottom-up organizational processes that are not required in visual imagery. Nevertheless, we note that if a familiar object is seen under degraded conditions (i.e., an object is partially occluded or an object is seen under poor lighting conditions; see Ganis, Schendan, & Kosslyn, 2007), then top-down processing is likely to be used in visual perception. In such circumstances, visual mental imagery would not necessarily be more disrupted than perception following damage to brain areas involved in top-down processing.

These conjectures are consistent with the fact that not all of the same brain areas are activated during visual mental imagery and visual perception (Ganis et al., 2004; Kosslyn et al., 1997). For example, Ganis et al. (2004) found less overlap in activation during imagery and perception in the occipital and temporal lobes than in the frontal and parietal lobes-a result that they interpreted as indicating that perception relies in part on bottom-up organizational processes that are not used as extensively in imagery. …