The main aims of this investigation were (1) to determine the effect of aging on selective attention function in dogs and (2) to establish specific factors that underlie the attention deficits. To this end, we hypothesized that reduced cognitive resources, an inability to ignore distracting information and/or reduced visual processing speed, would be three fundamental processes central to driving the decline in attention function.
To assess selective attention, we first trained animals to discriminate between two objects and then tested the animals with the one positive object and with zero, one, two, or three replicates of the negative object. The underlying rationale was that the task was essentially a visual search task requiring the subjects to locate the correct object. This hypothesis would predict that the greater the number of replicates of the incorrect objects, the poorer the animals would perform, as reflected by increased reaction time and decrease in ratio of correct response. Results from this investigation show that aging impairs response accuracy in the visual search task described here. The likelihood that three specific factors contribute in part to this impairment is discussed in detail further.
It has been suggested by several groups that selective attention ability depends on cognitive and attentional resources (Groth and Allen 2000
; Lavie and Tsal 1994
). Older subjects, who have reduced processing resources, have therefore been reported to perform poorly on two-choice visual tasks compared to younger subjects (Madden et al. 1992
). Our findings replicate this effect in aged versus young dogs. Senior dogs showed a robust increase in the number of errors on the two-choice discrimination of the task compared to young and old dogs. The outcome measure of the two-choice discrimination task determined the general visual learning ability of the dogs and confirms the hypothesis that aging impairs learning ability in dogs. This finding is consistent with previous findings in the dog (Tapp et al. 2003
) and makes a case for increased deficit in the cognitive reserve in aged dogs as compared to younger dogs.
The second factor was the inability to ignore distracting information: Several groups have shown that aging impairs performance in selective attention tasks (Groth and Allen 2000
; Nuechterlein et al. 1983
; Parasuraman 1979
; Parasuraman et al. 2002
; Rabbitt 1965
). Phases II and III of the task were designed to place a greater emphasis on selective attention and invoked preattentive or simultaneous processing abilities of visual stimuli in the subjects. The level of difficulty was also varied within each phase by changing the number of distracters and by increasing the similarity of distracter to target stimuli. As can be seen from the results, senior dogs were impaired in their ability to ignore distracting information in their field of view.
This is consistent with several results from functional neuroimaging and neurophysiological studies that demonstrate increases in neural activity and neural spiking in response to attended visual events over ignored stimuli in sensory cortical regions (Desimone and Duncan 1995
; Moran and Desimone 1985
; O'Craven et al. 1997
; Reynolds and Desimone 1999
). More recently, it was reported that older adults have reduced neural responses to sensory information (Peiffer et al. 2009
). Such functional differences in cortical activity between older and younger adults may provide the neural basis for the age-related deficit in ignoring irrelevant information. The effect of distraction in impairing performance in attention-based tasks has been attributed to weakened inhibitory control (Duchek et al. 1998
) which has also been described in the dog (Tapp et al. 2003
). Moreover, it had been reported, as early as 1965, that reduced inhibitory capacity results in vulnerability of older subjects to distracting stimuli (Rabbitt 1965
). In this study, we found a significant decrease in correct response to the target stimulus despite similar reaction times in senior dogs, suggesting that similar to human subjects, older dogs also show inhibitory decrements with age.
Although senior dogs differed significantly from adult dogs on error scores, the response latencies were not significantly different from younger dogs when assessed over all test trials. However, since the response latencies were not stable during the first three trials, we excluded these values from our analyses and found that when measured across the more stable trials, senior dogs take significantly longer to respond. Thus, reaction time, which measured the speed of processing in this test design, was greater for older dogs.
One major difference between our study and the others is the use of 3-s inspection times before allowing the animals to respond to stimulus presentation. This may have reduced the magnitude of difference in response time between groups. Alternatively, it is possible to speculate that the criterion described by Fisk and Schneider (1983
) is implicated. The authors suggested that simultaneous processing can be done automatically (preattentively), while serial processes require more processing time. Since this study was designed to test simultaneous and not serial processing, it is possible that the attentional resources of the dogs were not taxed enough to manifest as increase in reaction time across all trials. However, when only the more stable responses were taken into account, a robust effect was observed. Indeed, this is in agreement with the load theory (Lavie 1995
) which states that selective attention varies as a function of the load (Greenwood et al. 1997
The complex question of age-related decline in visual search ability evidently requires more work to precisely define the processes involved. However, this investigation clearly demonstrates that age-related impairments in selective attention is associated with, and indeed, may be derived from deficits in cognitive resources and an inability to ignore distracting information. It also provides strong evidence for the idea that age-induced deficits in selective attention occur not only in tasks which tap into working memory processes but also in less strenuous tests which require what may possibly be preattentive or automatic processing demands on cognitive resources.