Aftereffects in touch

• Prof. Astrid M.L. Kappers, VU University, Amsterdam, The Netherlands

• Dr. Wouter M. Bergmann Tiest, Rotterdam University of Applied Sciences, Rotterdam, The Netherlands

An aftereffect is the change in the perception of a (test) stimulus after prolonged stimulation with an (adaptation) stimulus. Usually, this change is in the negative direction, that is, in a direction opposite to that of the adaptation stimulus. Aftereffects are often fast and strong. A well-known example in vision is the waterfall illusion: when looking at trees after staring at a waterfall for a minute or more, the subsequently viewed trees seem to move upwards (Addams, 1834; Swanston and Wade, 1994). Also touch is susceptible to strong aftereffects: temperature, roughness, shape, curvature, motion and size of an object all give rise to aftereffects in touch.

Temperature aftereffect

One of the first reports of an aftereffect came from the 17th-century philosopher John Locke (1690/1975). In his essay on human understanding, he describes how water of a certain temperature can be perceived as different by the two hands. Although he did not perform an actual experiment, the idea is that if one hand is placed for some time in a bucket of cold water and the other in a bucket of warm water, the lukewarm water in a third bucket will feel as warm to the hand coming from the cold water and as cold to the other hand. This is an aftereffect of temperature. Arnold and colleagues (1982) actually performed this experiment with adults and children. Both groups of participants experienced this aftereffect. They also report that about half of the children responded that the water temperature in the third bucket was really different for the two hands, instead of just feeling differently. This was confirmed by the answer to the follow-up question about what would be the perceived temperature if the bucket were rotated by 180°. The majority of the children expected that the perceived temperature would switch after the rotation. Most of the adults responded to both questions correctly.

Size aftereffect

Walker and Shea (1974) placed a narrow bar to the right of a participant and a wide bar to the left (or vice versa). The participant was asked to repeatedly grasp the left and right bars with one of his/her hands and judge their width. After an exploration period of 2 minutes, the two bars were replaced with two identical bars of intermediate width and the participant had to judge which of the two bars was the wider. A significant aftereffect was found: the bar that was placed at the side where during the exploration phase the wider bar had been, felt narrower than the bar on the other side. Exploring with one hand and testing with the other hand did not result in an aftereffect. In a subsequent study (Walker, 1978), the participants had to explore and judge the length instead of the width of the bars and this led to similar results. Both these aftereffects were termed contingent (i.e. dependent) aftereffects, as they depended on the location of the stimulus. A similar experiment was performed using two hands: one hand repeatedly explored the length of a long bar, the other hand explored a short bar. This also led to a significant aftereffect: when the two hands were subsequently presented with bars of intermediate length, the bar that was touched with the hand that previously touched the longer one felt shorter than the other.

Uznadze (1966) did a similar experiment with three-dimensional shapes, namely spheres. His participants were asked to repeatedly grasp spheres with their hands, always a small one on one side and a large one on the other side. After about 15 such grasps, the spheres were replaced with two identical spheres with a size in between that of the small and large ones. As in the width and length aftereffects, the size of the sphere that was grasped with the hand that previously grasped the large one was perceived as smaller than that grasped by the other hand. Kappers and Bergmann Tiest (2013) repeated this experiment in a more formal way. They showed that the strength of this aftereffect, expressed as the difference in perceived size of the spheres, was 24% of the size of the test sphere. They also showed that the grasping tetrahedra instead of spheres led to similar aftereffects (Kappers and Bergmann Tiest, 2014). Interestingly, Maravita (1997) showed that the same size aftereffect existed in a right-brain damaged patient. Although this patient did not consciously perceive the sphere presented to his left hand, the size of this sphere influenced the perception of the size of the sphere presented to his right hand. The occurrence of the aftereffect showed that somatosensory information that is not consciously perceived is still processed by the brain.

Curvature aftereffect

In 1933, Gibson published a study on the visual and kinesthetic perception of curved lines. He observed that in both modalities, the perception of curvature was strongly biased by previous exposure to a curved stimulus. In his kinesthetic experiment, blindfolded participants were instructed to run their fingers over a convexly curved piece of cardboard for 3 minutes. After this adaptation period, they were asked to move their fingers in the same manner along a straight edge. All the participants reported that the straight edge felt concave. Moreover, the participants also reported a gradual decrease of the perceived curvature during the 3 minutes exposure period.

Vogels and colleagues (1996) used solid three-dimensional shapes to study the aftereffect of touching a curved surface. Participants seated behind a curtain were required to place one of their hands on a curved conditioning shape. After a fixed time (usually just 5 s), participants lifted their hand and the experimenter presented them with a differently curved test surface. Subsequently, they had to respond as quickly as possible whether this test surface was convex or concave. Such trials were repeated for many different combinations of adaptation and test curvatures. Even after such a short adaptation phase, significant aftereffects were found: the perceived curvature of the test stimulus was strongly shifted in a direction opposite to that of the conditioning curvature. In other words, after adapting to a concave stimulus, a flat surface feels convex, and vice versa. In a second experiment, the length of the conditioning period was varied. Interestingly, after adapting for only 2 s to a curvature, the perception of the subsequently touched curved surface was changed. The strength of the aftereffect reached its maximum after about 10 s; longer adaptation periods did not increase the strength of the effect. In the third experiment, the duration of the aftereffect was tested. Instead of touching the test surface immediately after the adaptation phase, participants had to wait for up to 80 s. Although the strength of the aftereffect diminished with time, a significant aftereffect was still present after 40 s. Thus, touching a curved surface only briefly has a relatively long effect on the perception of objects that are touched next. Indeed, when touching three curved surfaces in a row, the perceived curvature of the third is influenced by the curvatures of both the first and the second touched surfaces (Vogels et al., 2001).

In another study, Vogels et al. (1997) investigated the origin of this curvature aftereffect. In the first experiment, they varied the hand activity of the participant in the period of 5, 20 or 40 s between the adaptation and the test phases: participants either held their hand passively in the air, made a fist, or repeatedly bent and stretched their fingers. The idea behind these conditions was that the stimulation of cutaneous receptors would be quite different in the three conditions. Therefore, if the cutaneous receptors played an important role in the aftereffect, the rate of decay of the aftereffect would strongly depend on the intervening activity. However, the decay rates in the various conditions were quite similar, and so the conclusion was that the role of the cutaneous receptors (i.e. the peripheral level) in the aftereffect is at most minor. Thus the origin of the aftereffect had to be at a higher processing level. In the second experiment, they tested whether the aftereffect transferred to the other hand. If that were the case, the origin of the aftereffect would be highly central. In this experiment, participants adapted to a curved surface with one hand and the test was presented to the other hand. Only two participants took part in this study, but neither of them showed a transfer of the aftereffect to the other hand. Thus, the origin of the aftereffect could not be highly central, either.

Van der Horst and colleagues (2008a) showed that a curvature aftereffect also occurs when the adaption surface is only touched with a static finger. They also showed a partial but significant transfer of the aftereffect between fingers, not only between those of the same hand, but also across hands. This was again strong evidence that the aftereffect does not originate at the peripheral level of the cutaneous receptors in the skin. Moreover, because the partial transfer also occurred between the fingers of different hands, the origin of this particular curvature aftereffect necessarily lies at a high central level, probably at the level of the somatosensory cortex. In an other study (Van der Horst et al., 2008b), they investigated the curvature aftereffect and its possible transfer when fingers either actively or passively touched curved stimuli. In the active dynamic condition, participants moved their fingers over a curved surface. This condition is reminiscent of that of Gibson (1933), except that the stimuli were presented vertically instead of horizontally and the adaptation phase was much shorter (three back-and-forth movements instead of 3 minutes of exploration). In the passive dynamic condition, the stimulus moved repeatedly from left to right and back underneath the static finger. In both conditions, a full transfer to the other hand was found. Thus a dynamically obtained aftereffect has to originate at a high level. It was also found that the strength of the aftereffect was substantially larger in the active condition. From this it followed that the aftereffect and thus also the representation of curvature, depends on the exploration mode, and that kinesthetic information plays an additional role in the aftereffect. A detailed overview of the various experimental conditions in the above-cited papers can be found in Kappers (2011).

Denisova and colleagues (2014) investigated whether an aftereffect also occurred when the curved surface was touched with a tool instead of directly with the hand or fingers. Their virtual stimuli were simulated versions of the real ones used by Van der Horst et al. (2008b). They found both an aftereffect and a partial transfer of the aftereffect to the other hand, which confirms that in the dynamic case, kinesthetic information plays a major role in the origin of the aftereffect.

Shape aftereffect

As mentioned above, Kappers and Bergmann Tiest (2014) showed that size aftereffects occurred for both spheres and tetrahedra. They also tested “mixed” conditions: adaptation to one type of shape, sphere or tetrahedron, and testing with the other type. Interestingly, in these conditions the aftereffects were much smaller and for many participants they did not even exist. They concluded that the “size aftereffect” for three-dimensional shapes, should more properly be termed “size-shape aftereffect”. As shape information is not processed at a peripheral level, this finding suggests that higher cortical areas are involved in this aftereffect.

Inspired by findings in vision, a rather different shape aftereffect was investigated by Matsumiya (2012). His participants had to haptically explore face masks with different expressions (sad, happy or neutral). He ran two conditions. In the non-adaptation condition, the participants had to explore one of the three test faces and subsequently make a decision about whether the face had a sad or happy expression. In the adaptation condition, participants first explored either the happy or the sad face mask for 20 s and subsequently had to judge the expression of the neutral face as either happy or sad. He then compared the percentages “sad” responses to the neutral face mask in the different conditions. Without adaptation, the percentage of “sad” faces was 50%. With adaptation to the happy face, the percentage of “sad” responses to the neutral face increased to about 90%, whereas after adaptation to the sad face, this percentage decreased to about 35% (percentages estimated from his figure 2). The main conclusion that he drew from these results is that probably the face aftereffect originates at higher cortical areas, such as the inferior frontal gyrus, inferior parietal lobe, and/or superior temporal sulcus, as that is where haptic facial information is processed (Matsumiya, 2012; Kitada et al., 2010).

Tactile motion aftereffect

Motion of a stimulus over the skin may give rise to a tactile motion aftereffect, which is the impression that after removing the adaptation stimulus, a stimulus still moves over the skin. However, both the occurrence and the direction of this motion aftereffect critically depends on the precise experimental conditions, as not all studies have found effects (Watanabe et al., 2007). One of the first studies was rather informal, with participants (the author and colleagues) describing their introspective perceptions (Thalman, 1922). In some of the conditions, some or all of the participants experienced an aftereffect. Hazlewood (1971), using a rather sophisticated set-up, failed to find a tactile motion aftereffect. Only one of her 80 participants reported an aftereffect. Hollins and Favorov (1994) report that a surface with a smooth microtexture elicits the best aftereffects. Their five participants experienced a mostly negative (that is, in the opposite direction as in most other aftereffects) tactile motion aftereffect in most of the trials. The strength and the duration of the aftereffect increased with the duration of the adaptation phase (between 30 and 120 s). However, in a replication of this experiment, Lerner and Craig (2002) found that only about half of their 50 participants reported an aftereffect, which were mostly in positive or other directions. Using a different set-up, again about 50% reported an aftereffect, but this time mostly in a negative direction.

Watanabe and colleagues (2007) argued that in order to elicit a reliable tactile motion aftereffect, appropriate stimuli have to be used to stimulate the same mechanoreceptors in the adaptation and test phases. They used a set of three pins spaced 5 mm apart, that vibrated with a frequency of 30 Hz. By giving the pins a different phase, participants perceived apparent motion over their fingers. Such stimulation was used during an adaptation phase of 10 s. This resulted in robust tactile motion aftereffects in the opposite direction. In a number of sophisticated experimental conditions, Kuroki et al. (2012) found that the direction of the tactile motion aftereffect was determined by the environmental direction and not by the somatotopic direction. Moreover, they concluded that stimulation of peripheral receptors is essential for the occurrence of the aftereffect.

Planetta and Servos (2008) investigated, among others, the influence of the speed of the adapting motion. They found that duration, frequency and vividness of the aftereffect increased with speed. They found a tactile motion aftereffect in only about 50% of their trials of which the direction could be positive, negative or “other”. In a subsequent study (Planetta and Servos, 2010), they investigated which type(s) of mechanoreceptors are involved in the tactile motion aftereffect. They compared different sites of stimulation, namely the hand, the cheek and the forearm. A tactile motion aftereffect was most often reported after stimulation of the hand. From their findings they concluded that most likely the fast adapting type I receptors and the hair follicles are involved in the tactile motion aftereffect. The main conclusion from most of the above-mentioned studies is that tactile motion aftereffects are much harder to induce than visual motion aftereffects (for example, the above-mentioned waterfall illusion).

A different aftereffect of tactile motion is the tactile speed aftereffect: after an adaptation phase of exposure to a moving stimulus, a moving test stimulus is perceived as moving slower as compared to the same test stimulus presented to a non-adapted hand (McIntyre et al., 2012). This effect was independent of the direction of the adapting stimulus relative to the test direction. As peripheral afferents are direction sensitive, this independence of direction suggests that adaptation of these afferents cannot be the cause of the aftereffect: adaptation in their preferred direction should cause stronger adaptation and as a consequence, a direction sensitive aftereffect. As this was clearly not the case, the authors conclude that this aftereffect has to be of central origin.

Vibration aftereffect

Lederman and colleagues (1982) asked participants to place their finger for 10 minutes on a stimulus vibrating at either 20 or 250 Hz. After this adaptation period, the participants had to make magnitude estimates of vibrating stimuli with different intensities. In most cases, the magnitude estimates of the intensity of vibrating test stimuli decreased in comparison to estimates made without adaptation. The effects were stronger for the test stimuli with vibration frequencies equal to the adaptation vibration frequency. In the case of the 250 Hz adaptation and test vibration frequencies, the test stimuli with the lowest intensities were no longer detected. The authors also showed that such an adaptation to vibration does not influence the perceived roughness of stimuli. From this finding, they concluded that the sense of vibration is not the underlying mechanism for the perception of roughness. However, more recently, Hollins et al. (2001) found that adaptation to vibration does affect roughness discrimination performance of very fine textures (spatial periods below 100 μm), but not that of rougher textures. They conclude that in the former case, the peripheral Pacinian receptors play a major role.

Hahn (1966) showed that longer adaptation periods lead to stronger aftereffects, both with a method of absolute thresholds and a matching experiment. Even after an adaptation period of 25 minutes, the increase in the strength of the aftereffect was not yet saturated. After such a long adaptation period, a recovery period of 10 minutes was needed to return to values before adaptation.

Roughness aftereffect

Kahrimanovic and colleagues (2009) let participants adapt to a rough surface by making a back-and-forth scanning movement with a finger. The perceived roughness of the next surface was decreased in comparison to the roughness perceived with a non-adapted finger. Also the opposite occurred: after adaptation to a smooth surface, the perceived roughness of the next surface was increased. The authors argue that from these findings that this aftereffect is not caused at a peripheral level. If the aftereffect were due to overstimulation of certain types of mechanoreceptors, the aftereffect after adaptation to smooth and rough stimuli should be in the same direction and not in opposite directions, with a smaller effect for smooth stimuli as compared to rough stimuli. They hypothesize that the adaptation originates in the somatosensory cortex.

Weight aftereffect

De Mendoza (1979) asked participants in a series of trials to compare weights presented to the two hands. These weights were either equal or differed substantially. He found a significant aftereffect of weight: if two identical weights were presented after substantially different ones, the stimulus presented to the hand that in the previous trial held the heavier one, was perceived as lighter than the identical weight presented to the other hand. The author proposed to term this aftereffect the “gravimetric aftereffect”, as it was unclear whether it originated in proprioception, kinesthesis, tactile sensitivity or combinations thereof.

Cross-modal aftereffect

Many of the aftereffects mentioned above exist in both vision and touch. It is therefore of interest to investigate whether these aftereffects have a similar origin. One approach to this question is to test whether an aftereffect transfers from one modality to the other. Konkle and colleagues (2009) tested transfer of the motion aftereffect. They found that after adaptation to a visually moving stimulus, a tactile motion aftereffect could be measured. The illusion of motion was in a direction opposite to that of the inducing visual motion. They also found a visual motion aftereffect after adapting to tactile motion. This study provides strong evidence that processing of visual and tactile motion needs at least partially to take place in a common brain area. In a subsequent study, Konkle and Moore (2009) argue that the origin of aftereffects can lie in many different brain areas, depending on the type of processing.

Matsumiya (2013) found face aftereffects, the changed perception of emotional expressions (see above), in both touch and vision. He also found that these transferred both from vision to touch and vice versa. Again the conclusion had to be that face processing in vision and touch share representations in the brain.

Conclusion

Touch is susceptible to strong aftereffects: curvature, size, shape, roughness, vibrations, temperature and weight all lead to aftereffects. These aftereffects are always negative, that is, in a direction opposite to that of the adapting stimulus. Also motion sometimes induces aftereffects in touch, but the effects are usually less strong, not always found in all participants, and the direction can be positive, negative or even in other directions. For most of these aftereffects there exists evidence that they originate in higher brain areas and not by overstimulation of peripheral receptors.

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Sponsored by: Prof. Tony J. Prescott, Dept Psychology, Univ of Sheffield, UK

Reviewed by: Anonymous (via Prof. Vincent Hayward, Université Pierre et Marie Curie)

Reviewed by: Dr. Lynette Jones, Department of Mechanical Engineering, MIT

Accepted on: 2015-03-30 16:14:52 GMT