Rhesus monkeys know when they remember.

Humans are consciously aware of some memories and can make verbal reports about these memories. Other memories cannot be brought to consciousness, even though they influence behavior. This conspicuous difference in access to memories is central in taxonomies of human memory systems but has been difficult to document in animal studies, suggesting that some forms of memory may be unique to humans. Here I show that rhesus macaque monkeys can report the presence or absence of memory. Although it is probably impossible to document subjective, conscious properties of memory in nonverbal animals, this result objectively demonstrates an important functional parallel with human conscious memory. Animals able to discern the presence and absence of memory should improve accuracy if allowed to decline memory tests when they have forgotten, and should decline tests most frequently when memory is attenuated experimentally. One of two monkeys examined unequivocally met these criteria under all test conditions, whereas the second monkey met them in all but one case. Probe tests were used to rule out "cueing" by a wide variety of environmental and behavioral stimuli, leaving detection of the absence of memory per se as the most likely mechanism underlying the monkeys' abilities to selectively decline memory tests when they had forgotten.

The parahippocampal region and object identification.

The hippocampus has long been thought to be critical for memory, including memory for objects. However, recent neuropsychological studies in nonhuman primates have indicated that other regions within the medial temporal lobe, specifically, structures in the parahippocampal region, are primarily responsible for object recognition and object identification. This article reviews the behavioral effects of removal of structures within the parahippocampal region in monkeys, and cites relevant work in rodents as well. It is argued that the perirhinal cortex, in particular, contributes to object identification in at least two ways: (i) by serving as the final stage in the ventral visual cortical pathway that represents stimulus features, and (ii) by operating as part of a network for associating together sensory inputs within and across sensory modalities.

Timing behaviour of black-capped chickadees (Parus atricapillus).

In Experiment 1 the behaviour of black-capped chickadees timing intervals of 12.5 or 37.5 s was studied using the peak procedure. Average rate of responding peaked near the trained FI on test trials, while the spread of the response distribution was larger for the longer FI. On individual trials, chickadees showed a break-run-break pattern of abrupt changes in rate of responding. These results, plus the pattern of means, standard deviations, and correlations found in start times, stop times, and durations of runs, were similar to those found in pigeons and rats. This suggests that birds and rodents use similar timing mechanisms. In Experiment 2, chickadees were tested with an interrupted FI signal. On such 'gap' trials, the chickadees, like pigeons but unlike rats tested under similar parameters, ignored the signal time elapsed prior to the FI interruption and reset the interval clock.

Spatial responses to light in mice with severe retinal degeneration.

It is known that mice homozygous for the retinal degeneration (rd) mutation are able to synchronize their circadian rhythms to light-dark cycles. In the present experiments mice were given a choice of a dark or an illuminated living and nesting area. C3H, CBA and C57 rd/rd mice spent more time in the dark than in the illuminated area. Also, they spent as much time in the dark area as did wildtype controls. This shows that, despite advanced retinal degeneration, light can be used to control behaviour in space as well as in time. This was true of mutant mice over a year old, when retinal degeneration is very severe, and also of a transgenic strain of mice whose rods are destroyed as they begin to develop in the first few weeks after birth.

Hippocampus and memory in a food-storing and in a nonstoring bird species.

Food-storing birds maintain in memory a large and constantly changing catalog of the locations of stored food. The hippocampus of food-storing black-capped chickadees (Parus atricapillus) is proportionally larger than that of nonstoring dark-eyed juncos (Junco hyemalis). Chickadees perform better than do juncos in an operant test of spatial non-matching-to-sample (SNMTS), and chickadees are more resistant to interference in this paradigm. Hippocampal lesions attenuate performance in SNMTS and increase interference. In tests of continuous spatial alternation (CSA), juncos perform better than chickadees. CSA performance also declines following hippocampal lesions. By itself, sensitivity of a given task to hippocampal damage does not predict the direction of memory differences between storing and nonstoring species.

Hippocampal lesions impair memory for location but not color in passerine birds.

The effects of hippocampal complex lesions on memory for location and color were assessed in black-capped chickadees (Parus atricapillus) and dark-eyed juncos (Junco hyemalis) in operant tests of matching to sample. Before surgery, most birds were more accurate on tests of memory for location than on tests of memory for color. Damage to the hippocampal complex caused a decline in memory for location, whereas memory for color was not affected in the same birds. This dissociation indicates that the avian hippocampus plays an important role in spatial cognition and suggests that this brain structure may play no role in working memory generally.

Effects of photoperiod on food-storing and the hippocampus in birds.

Birds that store food have a relatively large hippocampus compared to non-storing species. The hippocampus shows seasonal differences in neurogenesis and volume in black-capped chikadees (Parus atricapillus) taken from the wild at different times of year. We compared hippocampal volumes in black-capped chickadees captured at the same time but differing in food-storing behaviour because of manipulations of photoperiod in the laboratory. Differences in food-storing behaviour were not accompanied by differences in the volume of the hippocampus. Hippocampal volumes also did not differ between two groups of a non-food-storing control species, house sparrows (Passer domesticus), exposed to the same conditions as the chickadees.

Hippocampal volume and food-storing behavior are related in parids.

The size of the hippocampus has been previously shown to reflect species differences and sex differences in reliance on spatial memory to locate ecologically important resources, such as food and mates. Black-capped chickadees (Parus atricapillus) cached more food than did either Mexican chickadees (P. sclateri) or bridled titmice (P. wollweberi) in two tests of food storing, one conducted in an aviary and another in smaller home cages. Black-capped chickadees were also found to have a larger hippocampus, relative to the size of the telencephalon, than the other two species. Differences in the frequency of food storing behavior among the three species have probably produced differences in the use of hippocampus-dependent memory and spatial information processing to recover stored food, resulting in graded selection for size of the hippocampus.