Diminished Top-Down Control Underlies a Visual Imagery Deficit in Normal Aging

Abstract

Mental imagery is involved in a wide variety of cognitive abilities, including reasoning, spatial navigation, and memory. Cognitive aging is associated with impairments in these abilities, suggesting diminished fidelity of mental images in older adults may be related to diverse cognitive deficits. However, an age-related deficit in mental imagery and its role in memory impairment is still a matter of debate. Previous human fMRI studies demonstrated that visual imagery activates representations in category-selective visual cortex via top-down control mechanisms. Here, we use fMRI to show that normal aging is associated with diminished selectivity of visual cortex activation during visual imagery, with a corresponding reduction in the selectivity of functional connections between prefrontal cortex and visual cortices. Moreover, a relationship between reduced imagery selectivity and visual memory in older adults was established. These results reveal that aging disrupts neural networks that subserve mental imagery and offers evidence of this as a factor in age-related memory decline.

Reversal of cortical information flow during visual imagery as compared to visual perception

“…We hypothesized that during visual perception bottom-up connectivity from early visual areas to higher order cortices would be predominant; whereas during visual mental imagery, higher order areas would lead the recruitment of early visual cortices in a top-down manner. This idea is consistent with current notions of visual imagery and perception and with indirect experimental evidence (Ganis and Schendan, 2008; Ishai et al, 2000; Kalkstein et al, 2011; Kosslyn, 2005; Mechelli et al, 2004). …”

“…It has been proposed that, while brain forward connections convey information from the outside world, backward connections might have a dominant role during the forming of mental images in the absence of external bottom-up inputs (Ganis and Schendan, 2008; Ishai et al, 2000; Kalkstein et al, 2011; Kosslyn, 2005). Despite the relevance of top-down and bottom-up dynamics for the understanding of the generative mechanisms of visual mental representations (Corbetta and Shulman, 2002; Friston, 2002; Kosslyn, 2005), a direct quantitative comparison of the directionality of neural signal flow during visual perception and imagery is still missing.…”

How ageing changes the mnemonic bias of visual behaviour

“…if ageing leads to attenuation and/or slowing of the feedback processes between a fronto-thalamic network and visual cortex that are critical for memory biasing of attention. This interpretation is also in keeping with the view that ageing impairs the neural manifestation of mental imagery in sensory cortices including a reduction in functional connectivity between prefrontal regions and the relevant visual cortical area for imagery (Kalkstein et al 2011).…”

Cholinergic enhancement of functional networks in older adults with mild cognitive impairment

“…Functional connectivity maps of the regions of interest were generated by extracting beta values for every stage of every trial and correlating these values using the beta series correlation method . The beta series correlation method was a functional connectivity analysis approach that utilized trial‐by‐trial variability to measure covariance in activity between spatially disparate regions, and offered a powerful tool for assessing network interactions during substages of a trial . Single‐subject z ‐transformed maps were normalized to the Montreal Neurologic Institute (MNI) space (2mm voxels) and Gaussian smoothed (5mm full width at half maximum) for the group analysis.…”