The size-weight illusion induced through human echolocation.

Certain blind individuals have learned to interpret the echoes of self-generated sounds to perceive the structure of objects in their environment. The current work examined how far the influence of this unique form of sensory substitution extends by testing whether echolocation-induced representations of object size could influence weight perception. A small group of echolocation experts made tongue clicks or finger snaps toward cubes of varying sizes and weights before lifting them. These echolocators experienced a robust size-weight illusion. This experiment provides the first demonstration of a sensory substitution technique whereby the substituted sense influences the conscious perception through an intact sense.

A blind human expert echolocator shows size constancy for objects perceived by echoes.

Some blind humans make clicking noises with their mouth and use the reflected echoes to perceive objects and surfaces. This technique can operate as a crude substitute for vision, allowing human echolocators to perceive silent, distal objects. Here, we tested if echolocation would, like vision, show size constancy. To investigate this, we asked a blind expert echolocator (EE) to echolocate objects of different physical sizes presented at different distances. The EE consistently identified the true physical size of the objects independent of distance. In contrast, blind and blindfolded sighted controls did not show size constancy, even when encouraged to use mouth clicks, claps, or other signals. These findings suggest that size constancy is not a purely visual phenomenon, but that it can operate via an auditory-based substitute for vision, such as human echolocation.

Counting on the motor system: rapid action planning reveals the format- and magnitude-dependent extraction of numerical quantity.

Symbolic numbers (e.g., "2") acquire their meaning by becoming linked to the core nonsymbolic quantities they represent (e.g., two items). However, the extent to which symbolic and nonsymbolic information converges onto the same internal core representations of quantity remains a point of considerable debate. As nearly all previous work on this topic has employed perceptual tasks requiring the conscious reporting of numerical magnitudes, here we question the extent to which numerical processing via the visual-motor system might shed further light on the fundamental basis of how different number formats are encoded. We show, using a rapid reaching task and a detailed analysis of initial arm trajectories, that there are key differences in how the quantity information extracted from symbolic Arabic numerals and nonsymbolic collections of discrete items are used to guide action planning. In particular, we found that the magnitude derived from discrete dots resulted in movements being biased by an amount directly proportional to the actual quantities presented whereas the magnitude derived from numerals resulted in movements being biased only by the relative (e.g., larger than) quantities presented. In addition, we found that initial motor plans were more sensitive to changes in numerical quantity within small (1-3) than large (5-15) number ranges, irrespective of their format (dots or numerals). In light of previous work, our visual-motor results clearly show that the processing of numerical quantity information is both format and magnitude dependent.

Connecting the dots: object connectedness deceives perception but not movement planning.

The perceptual system parses complex scenes into discrete objects. Parsing is also required for planning visually guided movements when more than one potential target is present. To examine whether visual perception and motor planning use the same or different parsing strategies, we used the connectedness illusion, in which observers typically report seeing fewer targets if pairs of targets are connected by short lines. We found that despite this illusion, when observers are asked to make speeded reaches toward targets in such displays, their reaches are unaffected by the presence of the connecting lines. Instead, their movement plans, as revealed by their movement trajectories, are influenced by the number of potential targets irrespective of whether connecting lines are present or not. This suggests that scene parsing for perception depends on mechanisms that are distinct from those that allow observers to plan rapid and efficient target-directed movements in situations with multiple potential targets.

One to four, and nothing more: nonconscious parallel individuation of objects during action planning.

Much of the current understanding about the capacity limits on the number of objects that can be simultaneously processed comes from studies of visual short-term memory, attention, and numerical cognition. Consistent reports suggest that, despite large variability in the perceptual tasks administered (e.g., object tracking, counting), a limit of three to four visual items can be independently processed in parallel. In the research reported here, we asked whether this limit also extends to the domain of action planning. Using a unique rapid visuomotor task and a novel analysis of reach trajectories, we demonstrated an upper limit to the number of targets that can be simultaneously encoded for action, a capacity limit that also turns out to be no more than three to four. Our findings suggest that conscious perceptual processing and nonconscious movement planning are constrained by a common underlying mechanism limited by the number of items that can be simultaneously represented.

Visual salience dominates early visuomotor competition in reaching behavior.

In this study, we investigated whether visual salience influences the competition between potential targets during reach planning. Participants initiated rapid pointing movements toward multiple potential targets, with the final target being cued only after the reach was initiated. We manipulated visual salience by varying the luminance of potential targets. Across two separate experiments, we demonstrate that initial reach trajectories are directed toward more salient targets, even when there are twice as many targets (and therefore twice the likelihood of the final target appearing) on the opposite side of space. We also show that this salience bias is time-dependent, as evidenced by the return of spatially averaged reach trajectories when participants were given an additional 500-ms preview of the target display prior to the cue to move. This study shows both when and to what extent task-irrelevant luminance differences affect the planning of reaches to multiple potential targets.

Enhanced auditory spatial localization in blind echolocators.

Echolocation is the extraordinary ability to represent the external environment by using reflected sound waves from self-generated auditory pulses. Blind human expert echolocators show extremely precise spatial acuity and high accuracy in determining the shape and motion of objects by using echoes. In the current study, we investigated whether or not the use of echolocation would improve the representation of auditory space, which is severely compromised in congenitally blind individuals (Gori et al., 2014). The performance of three blind expert echolocators was compared to that of 6 blind non-echolocators and 11 sighted participants. Two tasks were performed: (1) a space bisection task in which participants judged whether the second of a sequence of three sounds was closer in space to the first or the third sound and (2) a minimum audible angle task in which participants reported which of two sounds presented successively was located more to the right. The blind non-echolocating group showed a severe impairment only in the space bisection task compared to the sighted group. Remarkably, the three blind expert echolocators performed both spatial tasks with similar or even better precision and accuracy than the sighted group. These results suggest that echolocation may improve the general sense of auditory space, most likely through a process of sensory calibration.

Parahippocampal cortex is involved in material processing via echoes in blind echolocation experts.

Some blind humans use sound to navigate by emitting mouth-clicks and listening to the echoes that reflect from silent objects and surfaces in their surroundings. These echoes contain information about the size, shape, location, and material properties of objects. Here we present results from an fMRI experiment that investigated the neural activity underlying the processing of materials through echolocation. Three blind echolocation experts (as well as three blind and three sighted non-echolocating control participants) took part in the experiment. First, we made binaural sound recordings in the ears of each echolocator while he produced clicks in the presence of one of three different materials (fleece, synthetic foliage, or whiteboard), or while he made clicks in an empty room. During fMRI scanning these recordings were played back to participants. Remarkably, all participants were able to identify each of the three materials reliably, as well as the empty room. Furthermore, a whole brain analysis, in which we isolated the processing of just the reflected echoes, revealed a material-related increase in BOLD activation in a region of left parahippocampal cortex in the echolocating participants, but not in the blind or sighted control participants. Our results, in combination with previous findings about brain areas involved in material processing, are consistent with the idea that material processing by means of echolocation relies on a multi-modal material processing area in parahippocampal cortex.

The role of head movements in the discrimination of 2-D shape by blind echolocation experts.

Similar to certain bats and dolphins, some blind humans can use sound echoes to perceive their silent surroundings. By producing an auditory signal (e.g., a tongue click) and listening to the returning echoes, these individuals can obtain information about their environment, such as the size, distance, and density of objects. Past research has also hinted at the possibility that blind individuals may be able to use echolocation to gather information about 2-D surface shape, with definite results pending. Thus, here we investigated people's ability to use echolocation to identify the 2-D shape (contour) of objects. We also investigated the role played by head movements--that is, exploratory movements of the head while echolocating--because anecdotal evidence suggests that head movements might be beneficial for shape identification. To this end, we compared the performance of six expert echolocators to that of ten blind nonecholocators and ten blindfolded sighted controls in a shape identification task, with and without head movements. We found that the expert echolocators could use echoes to determine the shapes of the objects with exceptional accuracy when they were allowed to make head movements, but that their performance dropped to chance level when they had to remain still. Neither blind nor blindfolded sighted controls performed above chance, regardless of head movements. Our results show not only that experts can use echolocation to successfully identify 2-D shape, but also that head movements made while echolocating are necessary for the correct identification of 2-D shape.

Shape-specific activation of occipital cortex in an early blind echolocation expert.

We have previously reported that an early-blind echolocating individual (EB) showed robust occipital activation when he identified distant, silent objects based on echoes from his tongue clicks (Thaler, Arnott, & Goodale, 2011). In the present study we investigated the extent to which echolocation activation in EB's occipital cortex reflected general echolocation processing per se versus feature-specific processing. In the first experiment, echolocation audio sessions were captured with in-ear microphones in an anechoic chamber or hallway alcove as EB produced tongue clicks in front of a concave or flat object covered in aluminum foil or a cotton towel. All eight echolocation sessions (2 shapes×2 surface materials×2 environments) were then randomly presented to him during a sparse-temporal scanning fMRI session. While fMRI contrasts of chamber versus alcove-recorded echolocation stimuli underscored the importance of auditory cortex for extracting echo information, main task comparisons demonstrated a prominent role of occipital cortex in shape-specific echo processing in a manner consistent with latent, multisensory cortical specialization. Specifically, relative to surface composition judgments, shape judgments elicited greater BOLD activity in ventrolateral occipital areas and bilateral occipital pole. A second echolocation experiment involving shape judgments of objects located 20° to the left or right of straight ahead activated more rostral areas of EB's calcarine cortex relative to location judgments of those same objects and, as we previously reported, such calcarine activity was largest when the object was located in contralateral hemispace. Interestingly, other echolocating experts (i.e., a congenitally blind individual in Experiment 1, and a late blind individual in Experiment 2) did not show the same pattern of feature-specific echo-processing calcarine activity as EB, suggesting the possible significance of early visual experience and early echolocation training. Together, our findings indicate that the echolocation activation in EB's occipital cortex is feature-specific, and that these object representations appear to be organized in a topographic manner.

Short-term motor plasticity revealed in a visuomotor decision-making task.

Selecting and executing an action toward only one object in our complex environments presents the visuomotor system with a significant challenge. To overcome this problem, the motor system is thought to simultaneously encode multiple motor plans, which then compete for selection. The decision between motor plans is influenced both by incoming sensory information and previous experience-which itself is comprised of long-term (e.g. weeks, months) and recent (seconds, minutes, hours) information. In this study, we were interested in how the recent trial-to-trial visuomotor experience would be factored into upcoming movement decisions made between competing potential targets. To this aim, we used a unique rapid reaching task to investigate how reach trajectories would be spatially influenced by previous decisions. Our task required subjects to initiate speeded reaches toward multiple potential targets before one was cued in-flight. A novel statistical analysis of the reach trajectories revealed that in cases of target uncertainty, subjects initiated a spatially averaged trajectory toward the midpoint of potential target locations before correcting to the selected target location. Interestingly, when the same target location was consecutively cued, reaches were biased toward that location on the next trial and this effect accumulated across trials. Beyond providing supporting evidence that potential reach locations are encoded and compete in parallel, our results strongly suggest that this motor competition is biased by recent trial history.

Reaching for the unknown: multiple target encoding and real-time decision-making in a rapid reach task.

Decision-making is central to human cognition. Fundamental to every decision is the ability to internally represent the available choices and their relative costs and benefits. The most basic and frequent decisions we make occur as our motor system chooses and executes only those actions that achieve our current goals. Although these interactions with the environment may appear effortless, this belies what must be incredibly sophisticated visuomotor decision-making processes. In order to measure how visuomotor decisions unfold in real-time, we used a unique reaching paradigm that forced participants to initiate rapid hand movements toward multiple potential targets, with only one being cued after reach onset. We show across three experiments that, in cases of target uncertainty, trajectories are spatially sensitive to the probabilistic distribution of targets within the display. Specifically, when presented with two or three target displays, subjects initiate their reaches toward an intermediary or 'averaged' location before correcting their trajectory in-flight to the cued target location. A control experiment suggests that our effect depends on the targets acting as potential reach locations and not as distractors. This study is the first to show that the 'averaging' of target-directed reaching movements depends not only on the spatial position of the targets in the display but also the probability of acting at each target location.

Mutations in UDP-Glucose:sterol glucosyltransferase in Arabidopsis cause transparent testa phenotype and suberization defect in seeds.

In higher plants, the most abundant sterol derivatives are steryl glycosides (SGs) and acyl SGs. Arabidopsis (Arabidopsis thaliana) contains two genes, UGT80A2 and UGT80B1, that encode UDP-Glc:sterol glycosyltransferases, enzymes that catalyze the synthesis of SGs. Lines having mutations in UGT80A2, UGT80B1, or both UGT80A2 and UGT8B1 were identified and characterized. The ugt80A2 lines were viable and exhibited relatively minor effects on plant growth. Conversely, ugt80B1 mutants displayed an array of phenotypes that were pronounced in the embryo and seed. Most notable was the finding that ugt80B1 was allelic to transparent testa15 and displayed a transparent testa phenotype and a reduction in seed size. In addition to the role of UGT80B1 in the deposition of flavanoids, a loss of suberization of the seed was apparent in ugt80B1 by the lack of autofluorescence at the hilum region. Moreover, in ugt80B1, scanning and transmission electron microscopy reveals that the outer integument of the seed coat lost the electron-dense cuticle layer at its surface and displayed altered cell morphology. Gas chromatography coupled with mass spectrometry of lipid polyester monomers confirmed a drastic decrease in aliphatic suberin and cutin-like polymers that was associated with an inability to limit tetrazolium salt uptake. The findings suggest a membrane function for SGs and acyl SGs in trafficking of lipid polyester precursors. An ancillary observation was that cellulose biosynthesis was unaffected in the double mutant, inconsistent with a predicted role for SGs in priming cellulose synthesis.

A conserved functional role of pectic polymers in stomatal guard cells from a range of plant species.

Guard cell walls combine exceptional strength and flexibility in order to accommodate the turgor pressure-driven changes in size and shape that underlie the opening and closing of stomatal pores. To investigate the molecular basis of these exceptional qualities, we have used a combination of compositional and functional analyses in three different plant species. We show that comparisons of FTIR spectra from stomatal guard cells and those of other epidermal cells indicate a number of clear differences in cell-wall composition. The most obvious characteristics are that stomatal guard cells are enriched in phenolic esters of pectins. This enrichment is apparent in guard cells from Vicia faba (possessing a type I cell wall) and Commelina communis and Zea mays (having a type II wall). We further show that these common defining elements of guard cell walls have conserved functional roles. As previously reported in C. communis, we show that enzymatic modification of the pectin network in guard cell walls in both V. faba and Z. mays has profound effects on stomatal function. In all three species, incubation of epidermal strips with a combination of pectin methyl esterase and endopolygalacturonase (EPG) caused an increase in stomatal aperture on opening. This effect was not seen when strips were incubated with EPG alone indicating that the methyl-esterified fraction of homogalacturonan is key to this effect. In contrast, arabinanase treatment, and incubation with feruloyl esterase both impeded stomatal opening. It therefore appears that pectins and phenolic esters have a conserved functional role in guard cell walls even in grass species with type II walls, which characteristically are composed of low levels of pectins.

Identification of genes required for cellulose synthesis by regression analysis of public microarray data sets.

Coexpression patterns of gene expression across many microarray data sets may reveal networks of genes involved in linked processes. To identify factors involved in cellulose biosynthesis, we used a regression method to analyze 408 publicly available Affymetrix Arabidopsis microarrays. Expression of genes previously implicated in cellulose synthesis, as well as several uncharacterized genes, was highly coregulated with expression of cellulose synthase (CESA) genes. Four candidate genes, which were coexpressed with CESA genes implicated in secondary cell wall synthesis, were investigated by mutant analysis. Two mutants exhibited irregular xylem phenotypes similar to those observed in mutants with defects in secondary cellulose synthesis and were designated irx8 and irx13. Thus, the general approach developed here is useful for identification of elements of multicomponent processes.

Cell wall arabinan is essential for guard cell function.

Stomatal guard cells play a key role in the ability of plants to survive on dry land, because their movements regulate the exchange of gases and water vapor between the external environment and the interior of the plant. The walls of these cells are exceptionally strong and must undergo large and reversible deformation during stomatal opening and closing. The molecular basis of the unique strength and flexibility of guard cell walls is unknown. We show that degradation of cell wall arabinan prevents either stomatal opening or closing. This locking of guard cell wall movements can be reversed if homogalacturonan is subsequently removed from the wall. We suggest that arabinans maintain flexibility in the cell wall by preventing homogalacturonan polymers from forming tight associations.

Toward a systems approach to understanding plant cell walls.

One of the defining features of plants is a body plan based on the physical properties of cell walls. Structural analyses of the polysaccharide components, combined with high-resolution imaging, have provided the basis for much of the current understanding of cell walls. The application of genetic methods has begun to provide new insights into how walls are made, how they are controlled, and how they function. However, progress in integrating biophysical, developmental, and genetic information into a useful model will require a system-based approach.