Whilst the above studies largely approached recognition from a cognitive viewpoint, the use of neuroimaging in combination with traditional tasks has allowed inferences to be made about the brain regions involved in different aspects of memory. Coney and Macdonald (1988) investigated hemispheric differences in recognition by presenting stimuli to either the left or right visual field, thereby confining presentations to visual processing by one cerebral hemisphere or the other. Subsequent test presentations were either crossed (in the opposite field to the study presentation) or uncrossed (in the same field). The aim of the experiment was to determine whether hemispheric differences, for example the superior ability of the left hemisphere to process verbal material, contributed to lateral asymmetries in memory performance, and over what time
periods these asymmetries could be observed. Word stimuli were projected on a screen for 150ms, either to the left or right of a central fixation point.
No main effect of the visual field to which the stimulus was initially presented was found in reaction time data. However, the effect of the visual field to which probe items were presented was significant, with faster reaction times when words were presented in the right visual field. There was also a significant interaction between the target and probe visual fields with congruent target and probe visual fields (i.e. presentation for target and probe in the same visual field) producing faster correct reaction times than incongruent fields (target and probe in different visual fields). Results for different lags suggested that hemispheric interaction varied significantly over time. At a retention interval of 3 sec (lag 1) no differences were observed between the 4 presentation conditions, suggesting that perceptual matching accounts for responses, as opposed to comparisons with memory. After 12 sec (lag 4) left hemispheric processing was clearly dominant, probably as a result of the left hemisphere’s superior verbal processing abilities. The effect of crossed presentations only diverged significantly from uncrossed presentations at intervals of 32 sec (lag 8) and 96 sec (lag 32).
The results implied that memory traces were initially generated in both brain hemispheres in response to stimulus presentation, as no difference in recognition between crossed and uncrossed conditions was found, until a latency of 32 sec. These representations are likely to differ, however, in terms of their overall level of activation, with activation likely to be weaker in the indirectly-activated hemisphere than that directly indirectly-activated (contralateral to the visual field of presentation). Coney and Macdonald suggested two possible explanations for later hemispheric asymmetries. First, they suggested that decay of the two representations may have occurred to such an extent that the difference between direct and indirect traces affected retrieval time. Alternatively, once a trace had decayed to a critical level it may no longer have been effectively retrieved at all.
The fMRI study of Jessen and colleagues (Jessen et al., 2001) mentioned previously, found evidence of different activation patterns in participants when comparing encoding and recognition. Recognition of a test stimulus was associated with stronger activation of left parahippocampal and inferior frontal gyri than the initial study presentation of the same stimulus. These findings are consistent with the literature on amnesia and animal models of amnesia, which have found evidence of parahippocampal (Aggleton & Brown, 1999; Buffalo et al., 1999; Meunier, Bachevalier, Mishkin, & Murray, 1993) and frontal lobe (Bachevalier & Mishkin, 1986; Janowsky, Shimamura, Kritchevsky, & Squire, 1989; Owen, Sahakian, Semple, Polkey, & Robbins, 1995) involvement in recognition, and functional interaction between the two (Parker & Gaffan, 1998a).
Furthermore, when comparing the first and second repetitions of ‘old’ items, bilateral activation of frontal areas was stronger on the first repetition. This decreased frontal activity during the second repetition was thought to be an indication of the correspondingly reduced retrieval effort associated with recognition of items repeated a second time.
Whilst fMRI can give a good indication of the spatial profile of the anatomical substrate underlying the recognition process, better temporal resolution can be achieved with EEG measurements. Van Strien et al. (2005) examined changes in brain electrical activity during continuous recognition by EEG, and studied the resultant ERPs and induced band power, once again comparing novel items with their repetition, and repeated items at different levels of exposure. Recognition is associated with ‘old/new’ effects in the ERP, consisting of altered responses to repeated items compared to novel ones. Van Strien et al. found such an effect between 300 and 500 ms after exposure to a stimulus, when potentials for old items were associated with significantly greater positivity than those for new items. This effect was most pronounced around the midline parietal electrode position. In the time period 500-800 ms after
presentation, multiple repetitions of an item were associated with linearly increasing positivity, most pronounced at the midline central and fronto-central electrodes. Stronger memory for an item can therefore be related to increased positivity in this period. Similarly, induced bandpower (IBP) data showed evidence of higher bandpower in lower-2 alpha, theta, and delta bands for old items compared with new, and the induced delta activity was lessened with increasing repetitions in the period 375-750 ms after presentation. These effects constituted evidence, the authors suggested, for a dual-process interpretation of recognition, as familiarity was discernable from a graded recollection state dependent on repetitions of the stimulus.