Human peak shift: Analysis of the effects of three-stimulus discrimination training

In a series of studies, the effects of different types of intradimensional discrimination training on human auditory frequency generalization were examined. When subjects were trained with a single S− located on one or the other side of S+, postdiscrimination gradients were displaced away from S−. Subjects trained with two negative stimuli both on one side of S+ showed a greater extent of displacement with true peak shift. In a second experiment the procedures were repeated with two fixed amounts of training: either 12 or 42 training trials. Again the subjects trained with two negative stimuli showed more shift than those trained with one S−, and this effect was independent of amount of training. Experiment 3 showed increased peak shift when two positive stimuli surrounded a central S− as compared to groups with a single S+ and S−. The general conclusion is that training with more difficult, three-stimulus discrimination problems results in enhanced peak shift.     

Stimulus generalization and discrimination along the click-frequency (flutter) continuum in pigeons

Before tests for click-frequency generalization, pigeons had been reinforced for keypecks during one click frequency (S+). Some Ss received S+ training only, whereas other Ss also received unreinforced (S?) trials, during which the clicks were either absent (Experiments 1-3) or presented at some other frequency (faster or slower than S+: Experiment 4). When training included S+ trials only, birds responded approximately equally to all generalization test frequencies (0.0 to 53.5 pulses/sec, pps). Most Ss that had received both S+ and S? training trials responded fastest not during S+ but during click frequencies even further away from S? along the click-frequency dimension (peak shift). Complex bimodal gradients were obtained after training with S+ (1.6 pps) vs S? (0.0 pps); maximal responding generally occurred near S+ and at approximately 14.2 pps. Among other factors, the “nonorthogonality” of click absence (0.0 pps) to the click dimension seems crucially involved in producing these complex effects.     

Facilitation of learning of a simultaneous discrimination between rotated patterns by bumblebees

The failure to discriminate between a pattern consisting of four orthogonal bars and the same pattern rotated by 45° has been interpreted in the literature as evidence against pictorial representations in honeybees. This study determines whether prior training can facilitate the discrimination. In Experiment 1, one group of bumblebees was trained with a rewarding spiral pattern (S+) vs. its unrewarding 45° rotation (S−). A second group was trained with two different patterns, the spiral (S+) and the chevron (S−). Both groups were then tested with the spiral and its rotation, both of which were unrewarding. The first group failed in the discrimination while the second succeeded. The same was true when the training period was defined by choice frequency instead of time (Experiment 2). The facilitation due to prior experience was pattern specific (Experiments 3 and 4). These results show perceptual learning in bumblebees and weaken the case against pictorial representations.     

Target location probability effects in visual search: an effect of sequential dependencies

Target location probability was manipulated in a visual search task. When the target was twice as likely to appear on 1 side of the display as the other, manual button-press response times were faster (Experiment 1A) and first saccades were more frequently directed (Experiment 1B) to the more probable locations. When the target appeared with equal probability at each location in this search task, performance benefited from repetition of target location in the preceding trials (Experiment 2). When the trial sequence was constrained so that target location did not repeat within a series of 4 trials, there was no longer an advantage for more probable locations (Experiment 3). The authors conclude that the search benefits for more probable locations resulted from short-term target location repetitions.     

Stimulus generalization following extradimensional and intradimensional training in educable retarded children

Five related experiments investigating stimulus generalization following go/no-go discrimination training of educable retarded children are reported. Experiment 1 employed an Extradimensional paradigm in which generalization testing was on the hue dimension following training on an independent (orientation) dimension. Following True discrimination training only 25% of children showed a decremental stimulus generalization gradient on the hue dimension, though all children exhibited flat gradients in Pseudodiscrimination and S+ only control groups. An increase in difficulty of the orientation discrimination in Experiment 2 did not increase the number of decremental gradients. In Experiment 3, children who exhibited decremental gradients in Experiments 1 and 2 underwent further generalization testing with modified stimuli to establish a symmetrical gradient peaked at a hue S+ to be employed in Experiments 4 and 5. In these experiments an Intradimensional paradigm was employed with S+ and S? stimuli drawn from the hue dimension. Excitatory control by S+ and inhibitory control by S? were demonstrated, as were inhibitory consequences of S? such as peak and area shift.     

Distorted distance estimation induced by a self-relevant national boundary

Six experiments explored the possibility that the categorization of self versus not-self can distort visual estimations of distance. In Experiments 1-3, Americans overestimated the distance between a U.S. location and a foreign location relative to a visually equidistant U.S. location. In Experiment 4, Canadians overestimated foreign relative to Canadian locations. Experiment 5 showed that this effect occurs even when the domestic context is not physically contiguous. Experiment 6 showed that distance distortion occurs only when crossing one’s own border to a foreign location, not when crossing borders from one foreign location to another. Thus, crossing the psychological boundary between self and not-self creates a visual illusion that distorts on-line distance estimates.     

Inhibition of saccadic eye movements to locations in spatial working memory

Inhibition of return (IOR) refers to a bias against overt and covert attentional orienting toward previously attended locations. According to the reorienting hypothesis, IOR is generated when attention is withdrawn from the attended location and is prevented from “returning” to it. The present study investigated whether maintenance of attention at the cued location could affect the inhibition of oculomotor orienting to it. To preclude disengagement of attention, we asked participants to maintain the cued location in working memory. Maintenance of visuospatial information in memory has been shown to be accomplished through a sustained shift of spatial attention to a memorized location. Our results show that oculomotor IOR occurs at a particular location even when that location is kept in working memory (Experiment 1). Furthermore, we demonstrate that the mere act of maintenance of a location in working memory produces oculomotor inhibition similar to IOR (Experiments 2 and 3). We conclude that the oculomotor system is used for coding and maintaining locations in spatial working memory. In addition, we demonstrate that endogenous attention associated with maintenance of a location in working memory can be dissociated from the attention needed for execution of a saccadic eye movement.     

Spatial span under translation: A study of reference frames

The nature of the reference frame used to remember location sequences in a computer-presented version of spatial span was investigated by moving the template (a rectangular frame enclosing nine target squares) across the screen during presentation and/or during recall. Movement of the display during presentation substantially impaired memory in comparison with a stationary display (Experiment 1). However, there was no effect of template movement during recall (Experiment 2). In Experiments 3 and 4, the template was moved through the same screen locations during presentation and recall. When the extrinsic, or screen location, of each position was repeated identically on each trial but the sequence on the template varied, learning was not facilitated (Experiment 3). When the template sequences were repeated across trials but extrinsic location varied, the sequences were rapidly learned (Experiment 4). In this version of spatial span, location sequences appear to be encoded in an intrinsic frame of reference that is based on the template. Movement of the template during encoding impairs this process, possibly because concurrent attention shifts prevent the encoding of locations. The results are discussed with respect to recent studies of positional encoding in which multiple reference frames were available.     

Spatial representation by young infants: Categorization of spatial relations or sensitivity to a crossing primitive?

The spatial representation abilities of 3- to 4-month-old infants were examined in four experiments. Experiments 1 and 2 showed that infants familiarized with a diamond appearing in distinct locations to the left or right of a vertical bar subsequently preferred a stimulus depicting the diamond on the opposite side of the bar over a stimulus depicting the diamond in a novel location on the same side of the bar. Experiment 3 was a replication of Experiment 1, except that the bar was oriented at 45 degrees. In this instance, infants divided their attention between the stimulus depicting the diamond on the opposite side of the bar and the stimulus depicting the diamond in a novel location on the same side of the bar. Experiment 4 demonstrated that the results of Experiment 3 were not a consequence of a failure to process the diagonal bar. When considered with previous reports that infants can represent the categories of above and below (Quinn, 1994), the present results suggest that (1) infants can also represent the categories of left and right, and (2) performance cannot be interpreted as a response to an arbitrary crossing of one object relative to another. Although recent discussions of the relation between language and cognition have pointed to the ways in which spatial language influences spatial cognition (Bowerman & Levinson, 2001), the present findings are consistent with an influence in the opposite direction: Spatial cognition may in some instances shape spatial language.     

MATCHING-TO-SAMPLE PERFORMANCE IN RATS: A CASE OF MISTAKEN IDENTITY?

Three rats had previously acquired a simultaneous matching-to-sample performance with steady and blinking lights. In training, the sample stimulus had always appeared on the middle of three horizontally arranged keys with the comparison stimuli on the side keys. In Experiment 1, the sample stimulus appeared on any of the three keys with the comparison stimuli on the remaining two. The matching-to-sample performance broke down with variable sample and comparison locations; the sample stimulus did not control responding to the comparison stimuli when it appeared on a side key, but it retained control when it appeared on the middle key (as in training). In Experiment 2, the rats were trained with the sample always on the left key. When the sample appeared on either of the trained locations (left or middle key), it retained control for both locations. When the sample then appeared on any of the three keys, as in Experiment 1, sample control did not transfer to the untrained location (right key). The experiments demonstrate that training with fixed sample and comparison locations may establish spatial location as an additional controlling aspect of the stimuli displayed on the keys; stimulus location had become part of the definition of the controlling stimuli. The rats' performance seemed best described as specific discriminations involving the visual stimuli and their spatial locations rather than as identity matching.     

Peak shift but not range effects in recognition of faces

University students were trained to discriminate between two gray-scale images of faces that varied along a continuum from a unique face to an average face created by morphing. Following training, participants were tested without feedback for their ability to recognize the positive face (S+) within a range of faces along the continuum. In Experiments 1 and 4, the range of stimuli presented during testing was manipulated. In Experiment 2, participants viewed different ranges of faces during an adaptation period that followed training and preceded testing. In all experiments, generalization functions revealed peak shifts or area shifts (fewer “yes” responses to novel faces on the negative side of the S+), but no systematic effects of the test or adaptation range. Peak shift was found both for upright and inverted faces and occurred even if the orientation of the face was reversed between training and test. Using similar methods, either an area shift or range effect (but not both together) was demonstrated for line tilt stimuli (Experiment 3), and the appearance of these effects depended on instructions. It appears that peak shift and area shift are robust across many different kinds of stimuli, but range effects may not readily occur with complex multidimensional stimuli.     

Discrimination training of mirror-image stimuli with a delayed-prompt technique: some critical dimensions of extra-stimulus prompts

Research on fading and delay procedures has shown that extra-stimulus prompts frequently fail to help children learn difficult discriminations. The present study analyzed two delay conditions for an extra-stimulus prompt to help preschoolers discriminate mirror-image stimuli as a function of the configurations and locations of the prompts. All subjects were selected on their ability to discriminate the task stimuli in the presence of a third stimulus, a replica of the S+ and an arrow pointing to that stimulus; and on their inability to do so without these stimuli. Experiments 1 and 2 compared these prompt configurations (S+ replica, arrow) when presented equidistant from the task stimuli. Experiment 3 analyzed the contribution of the replica configuration in terms of its location, in between stimuli or immediately above the S+. Experiment 4 investigated the extent to which the results of the previous experiments could be influenced by the methodology for the assessment of prompt control. The results consistently demonstrated that most subjects did not learn the task unless the extra-stimulus prompt had the same configuration as the S+ and was located equidistant from both task stimuli.     

An investigation of peak shift and behavioral contrast for autoshaped and operant behavior.

Instrumental treadle press and nonreinforced key peck responses were monitored during discrimination training and generalization testing in pigeons on positive and negative reinforcement schedules. In Experiment 1, six pigeons pressed a treadle for food on a multiple variable-interval extinction schedule. In Experiment 2, three pigeons pressed a treadle to avoid shock on a multiple free-operant avoidance extinction schedule. Different color keylights signaled S+ and S- components. Some positive behavioral contrast occurred during discrimination training, but the effect was small. Pecking occurred to the S+ keylight in Experiment 1 but not in Experiment 2. On stimulus generalization tests, all subjects displayed a positive peak shift when pressing the treadle for food or to avoid shock. However, peak shift was not found for nonreinforced "autopecks" on the stimulus key, although an area shift was observed in Experiment 1. This is the first demonstration of peak shift for pigeons pressing treadles and the only reliable demonstration of peak shift when negative reinforcement maintained responding. These results, in combination with previous demonstrations of peak shift for rats pressing levers and pigeons pecking keys, indicate that peak shift is a general by-product of operant discrimination learning, since it occurs across a variety of the organisms, responses, and reinforcers.     

Memory for two types of spatial location: effects of instructions, age, and format

Three experiments, with old persons (59-80 years) and college students (17-30 years) in Experiments 1 and 2 and with college students (17-24 years) in Experiment 3, investigated the differences between two types of spatial location memory: memory for the location of individual items in an array and memory for occupied, as opposed to unoccupied, locations in an array. Young persons performed better than old persons on both measures of location memory. However, an effect of instructions (intentional vs. incidental for spatial location) was consistently obtained for memory for occupied, as opposed to unoccupied, locations, whereas no effect of instructions was obtained for memory for individual item locations. In addition, item location memory was superior for objects as opposed to matched words (Experiment 2), whereas occupied location memory was not affected by presentation format (Experiments 2 and 3). These differences indicate that spatial memory is a complex process whose properties are affected by variations in stimulus characteristics and task demands. It was concluded that the distinction of Hasher and Zacks (1979) between automatic and effortful processes is not adequate for understanding spatial memory. A recognition of the complex nature of spatial processing suggests a resolution of discrepancies in the literature based upon differences in stimulus characteristics, task demands, and the effectiveness of task-appropriate mnemonic strategies.     

The effect of negative stimulus presentations on observing-response rates

Theories of observing differ in predicting whether or not a signal for absence of reinforcement (S−) is capable of reinforcing observing responses. Experiments in which S− was first removed from and then restored to the procedure have yielded mixed results. The present experiments suggest that failure to control for the direct effect of presenting S− may have been responsible. Pigeons and operant procedures were used. Experiment 1 showed that presentations of S−, even when not contingent on observing, can raise the rate of an observing response that was reinforced only by presentations of a signal (S+) that accompanied a schedule of food delivery. Experiment 2 showed that this effect resulted from bursts of responding that followed offsets of S−. Experiment 3 showed that, when the presence of S− was held constant, lower rates occurred when S− was dependent on, rather than independent of, observing. These results support theories that characterize S− as incapable of reinforcing observing responses.     

Configural representations in spatial working memory

Two studies investigate the nature of representations in spatial working memory, directly addressing the question of whether people represent the configuration information above and beyond independent positional information when representing multiple sequentially presented locations. In Experiment 1, participants performed a location memory task in which they recalled the location of objects that were presented sequentially on a screen. Comparison of participants' data with simulated data modelled to represent independent positional representation of spatial locations revealed that participants represented the configural properties of shorter sequences (3 and 4 locations) but not of longer ones (5 and 7 locations). Experiment 2 employed a sequential recognition task in which participants were asked to judge whether two consequently presented spatial sequences were identical. These experiments confirmed sensitivity to configural properties of spatial sequences.     

What are human express saccades?

When a fixation point is removed 200 msec prior to target onset (the gap condition), human subjects are said to produce eye movements that have a short latency (80–120 msec), that form the early peak of a bimodal latency distribution, and that have been labeled “human express saccades” (see, e.g., Fischer, 1987; Fischer & Breitmeyer, 1987; Fischer & Ramsperger, 1984, 1986). In three experiments, we sought to obtain this express saccade diagnostic pattern in the gap condition, We orthogonally combined target location predictability with the presence versus absence of catch trials (Experiment 1). When target location was fixed and catch trials were not used, we found mostly anticipations. In the remaining conditions, where responses were under stimulus control, bimodality was not frequently observed, and, whether it was or not, latencies were not in the express saccade range. Using random target locations, we then varied stimulus luminance and the mode of stimulus presentation (LEDs vs. oscilloscope) in the gap and overlap (fixation is not removed) conditions (Experiment2). Bimodality was rarely observed, the gap effect (overlap minus gap reaction time) was additive with luminance, and only the brightest targets elicited saccades in the express range. When fixed locations and no catch trials were combined with latency feedback (Experiment 3), we observed many responses in the express saccade range and some evidence for bimodality, but the sudden introduction of catch trials revealed that many early responses were not under stimulus control. Humanscan make stimulus-controlled saccades that are initiated very rapidly (80–120 msec), but unless catch trials or choice reaction time is used, it is not possible to distinguish such saccades from anticipatory responses that are prepared in advance and timed to occur shortly after target onset. Because the express saccade diagnostic pattern is not a characteristic feature of human saccadic performance, we urge investigators to focus their attention on the robustgap effect     

Effects of spatial and selective attention on basic multisensory integration

When participants respond to auditory and visual stimuli, responses to audiovisual stimuli are substantially faster than to unimodal stimuli (redundant signals effect, RSE). In such tasks, the RSE is usually higher than probability summation predicts, suggestive of specific integration mechanisms underlying the RSE. We investigated the role of spatial and selective attention on the RSE in audiovisual redundant signals tasks. In Experiment 1, stimuli were presented either centrally (narrow attentional focus) or at 1 of 3 unpredictable locations (wide focus). The RSE was accurately described by a coactivation model assuming linear superposition of modality-specific activation. Effects of spatial attention were explained by a shift of the evidence criterion. In Experiment 2, stimuli were presented at 3 locations; participants had to respond either to all signals regardless of location (simple response task) or to central stimuli only (selective attention task). The RSE was consistent with task-specific coactivation models; accumulation of evidence, however, differed between the 2 tasks.     

More than meets the eye: the effect of planned fixations on scene representation

Scene memory frequently includes a swath of unseen layout beyond a photograph's boundaries (boundary extension [BE]; Intraub and Richardson, 1989). Might it be affected by the viewer's plan to shift fixation near a view boundary? When photographs were centrally fixated (500 msec), BE occurred following a 2-sec masked interval (Experiment 1). In Experiments 2-4, a cue during the first fixation signaled viewers to fixate an object near the left or right boundary. The picture was masked before the eyes landed. BE occurred on the cued side and on the top and bottom, but not on the uncued side. This relatively accurate performance on the uncued side suggests that inhibition of a movement to one side (in a competitive task) may also inhibit extrapolation of layout. BE on the to-be-fixated side, however, supports the idea that anticipatory representation of layout is an adaptive error that may aid the spatial integration of successive views.