Our study examined two key questions regarding age-related changes in the effect of spatial attention on sensory-evoked responses in visual cortex –– are there visual field differences in the age-related impairments in sensory processing that have been reported previously (e.g., Curran et al., 2001
), and do these impairments co-occur with changes in the executive control signals associated with visual spatial orienting? In this regard, we found that not only were sensory-evoked responses delayed in seniors specifically for unattended events in the left visual field as measured via latency shifts in the lateral occipital P1 elicited by visual targets, but seniors also showed amplitude reductions in the ADAN component elicited by cues directing attention to the left visual field. At the same time, seniors also had significantly higher error rates for targets presented in the left vs. right visual field. Taken together, our data thus converge on the conclusion that age-related changes in visual spatial attention involve both sensory-evoked and executive attentional control processes, and that these effects appear to be strongly associated with the left visual field.
If sensory-evoked responses are specifically delayed in seniors for unattended events in the left visual field, why might this be the case? When Curran et al. (2001)
reported a comparable effect in seniors based on data collapsed across visual fields, they attributed this latency shift –– which was found in the P1 component at the same ipsilateral electrode sites we report here –– to comparatively slower inter-hemispheric transfer speeds in seniors vs. young controls. While our findings are not inconsistent with this conclusion, that we found latency shifts in the P1 only for unattended events in the left visual field suggests that visual attention itself may also be a critical factor determining the speed of sensory responses in seniors. In particular, we found a delayed P1 latency when a target was presented in the left visual field but attention had been cued at the start of the trial to the right visual field. This suggests that seniors may have difficulty disengaging their visual spatial attention from the right visual field in response to the presentation of a target in the heretofore unattended left hemifield.
While our P1 data in seniors are thus consistent with selective problems in re-orienting attention to the left visual field, the ERPs time-locked to the attention-directing cues also point towards specific problems in the left visual field. To the point, both seniors and young controls showed comparable responses in the EDAN component regardless of visual field, a component which has been linked to the evaluation and interpretation of attention-directing cues (e.g., Jongen, Smulders, & Van der Heiden, 2007
). This suggests that the cues themselves were being equitably evaluated regardless of the participants’ age and the visual field to which the cue was directing attention. In contrast however, seniors showed amplitude reductions in the ADAN component elicited by cues to the left, relative to their responses to right visual field cues and relative to the ADANs for both cue directions in young controls. Given that the ADAN has been tied to the actual control of orienting visual spatial attention itself (e.g., Green, Conder, & McDonald, 2008
; Jongen, Smulders, & Van der Heiden, 2007
; Seiss et al., 2007
; but see Praamstra, Boutsen, & Humphreys, 2005
), this would indicate that not only do seniors have difficulty re-orienting attention to the left visual field in response to unexpected events, but that they also show selective decrements in volitionally orienting attention to the left visual field in response to directional cues.
Importantly, these age-related changes in orienting visual spatial attention to the left visual field, which we have construed as “impairments”, are not limited to the ERP measures we report here ––we also found that seniors were significantly less accurate in their behavioral responses to left vs. right visual field targets. While we can not make definitive causal links between our ERP and performance results, the collective evidence nevertheless suggests that seniors have greater difficulty in orienting their attention to the left vs. right visual field as measured not just by ERP indices of attentional orienting processes, but also in the overt responses they make to events in this visual hemifield. Given this conclusion, it’s also thus interesting to note that our results suggest that there was a speed-accuracy trade-off between seniors and young adults. Seniors were slower to respond to targets, but were also more accurate, relative to young adults. While we expected seniors to exhibit delayed reaction times, their increased accuracy suggests that it may not be due to a general slowing in cognitive processing, but rather an increase in conservativeness in order to avoid making errors (e.g., Ratcliff & McKoon, 2008
). While all participants were given the same instructions to respond both quickly and accurately, the performances differences that we observed between the two groups may be attributed to different internal priorities between seniors and young adults.
Performance issues aside, if our findings thus argue for age-related impairments in orienting visual spatial attention to the left visual field, what might be driving the constellation of effects we report here? Recent evidence has suggested that aging is specifically associated with a greater rate of decline in the right hemisphere relative to the left. For example, hemispheric asymmetries with age have been found for a variety of cognitive domains, such as memory span (e.g., Cherry et al., 2005
) and global processing (e.g., Lux et al., 2008
). Likewise, the neurocognitive processes associated with visual spatial attention also show strong laterality effects, such as is manifest in the strong prevalence of unilateral neglect in the left relative to the right visual field (e.g., Bublak, Redel, & Finke, 2006
; Reuter-Lorenz, Kinsbourne, & Moscovitch, 1990
), and the ability of the right hemisphere to orient attention to both visual hemifields but the left hemisphere only to the right visual field (e.g., Mangun et al., 1994
). Pairing these two lines of evidence together, it would suggest that the visual field asymmetries in age-related changes we report may be associated with specific age-related declines in right hemisphere processing.
In closing, we also note that there are several key control issues to consider regarding our data. First, it is apparent in our results that while normal attentional modulation of the P1 ERP component is evident in sites measured ipsilateral to the visual field of the target, the effects are notably reversed in both age groups at contralateral sites. Why? In perceptually easy tasks, such as the basic Posner cueing paradigm used in this study, cueing effects are shown primarily in ipsilateral sites (e.g., Handy & Mangun, 2000
; Kutas, Iragui, & Hillyard, 1994
; Onofrj et al., 2001
). Thus it is not surprising that we did not observe attention effects in the P1 at electrode sites contralateral to the visual field of the target. Second, the ERP waveforms of the senior participants are diminished, or “flattened out”. This attenuation of visual-evoked potentials (VEPs) and ERPs in seniors concurs with previous studies (e.g., Gilmore, 1995
; Kutas et al., 1994
; Nagamatsu et al., 2009
). Indeed, it has been suggested that the “severely impoverished” P1 in seniors agree with both fMRI evidence that seniors have decreased activity in primary visual cortices, and behaviour declines exhibited in seniors in visual perceptual abilities (Ceponiene et al., 2008
). Based on this, the morphology of ERPs in our study are consistent with those in previous studies using ERPs in seniors. Lastly, the ERP waveforms time-locked to the attention-directing cues in young adults are preceded by a large negative shift pre-baseline. This contingent negative variation (CNV) is related to the expectancy of stimulus onset (Walter et al., 1964
). Importantly, comparisons of the CNV across the lifespan have revealed larger expectancy effects in young adults versus seniors (e.g., Botzel et al., 2004
; Michalewski et al., 1980
). Therefore, it is not surprising that we find similar effects in our study.