A Review of Emerging Access Technologies for Individuals With Severe Motor Impairments

A brain-computer interface based on bilateral transcranial Doppler ultrasound

In this study, we investigate the feasibility of a BCI based on transcranial Doppler ultrasound (TCD), a medical imaging technique used to monitor cerebral blood flow velocity. We classified the cerebral blood flow velocity changes associated with two mental tasks - a word generation task, and a mental rotation task. Cerebral blood flow velocity was measured simultaneously within the left and right middle cerebral arteries while nine able-bodied adults alternated between mental activity (i.e. word generation or mental rotation) and relaxation. Using linear discriminant analysis and a set of time-domain features, word generation and mental rotation were classified with respective average accuracies of 82.9%10.5 and 85.7%10.0 across all participants. Accuracies for all participants significantly exceeded chance. These results indicate that TCD is a promising measurement modality for BCI research.

Camera-based microswitch technology for eyelid and mouth responses of persons with profound multiple disabilities: two case studies

These two studies assessed camera-based microswitch technology for eyelid and mouth responses of two persons with profound multiple disabilities and minimal motor behavior. This technology, in contrast with the traditional optic microswitches used for those responses, did not require support frames on the participants' face but only small color marks. The person involved in Study I had previously used optic sensors fixed on an eyeglasses' frame for detecting his eyelid- and mouth-opening responses. However, a deterioration of his head posture was making the correct location/use of this frame progressively more difficult. The person involved in Study II had previously been selected for a program relying on eyelid-closure responses and an optic sensor. Such a program however appeared difficult to implement given his sideward lying position and dystonic head movements. The new technology could be satisfactorily applied with both participants using mouth and eyelid opening with the first participant and eyelid closures with the second participant. Both participants had large increases in responding during the intervention periods (i.e., when their responses were followed by preferred stimulation). The findings are discussed in relation to the role of the new technology in helping persons with multiple disabilities and minimal motor behavior.

Validating an infrared thermal switch as a novel access technology

Background

Recently, a novel single-switch access technology based on infrared thermography was proposed. The technology exploits the temperature differences between the inside and surrounding areas of the mouth as a switch trigger, thereby allowing voluntary switch activation upon mouth opening. However, for this technology to be clinically viable, it must be validated against a gold standard switch, such as a chin switch, that taps into the same voluntary motion.

Methods

In this study, we report an experiment designed to gauge the concurrent validity of the infrared thermal switch. Ten able-bodied adults participated in a series of 3 test sessions where they simultaneously used both an infrared thermal and conventional chin switch to perform multiple trials of a number identification task with visual, auditory and audiovisual stimuli. Participants also provided qualitative feedback about switch use. User performance with the two switches was quantified using an efficiency measure based on mutual information.

Results

User performance (p = 0.16) and response time (p = 0.25) with the infrared thermal switch were comparable to those of the gold standard. Users reported preference for the infrared thermal switch given its non-contact nature and robustness to changes in user posture.

Conclusions

Thermal infrared access technology appears to be a valid single switch alternative for individuals with disabilities who retain voluntary mouth opening and closing.

Client-centred development of an infrared thermal access switch for a young adult with severe spastic quadriplegic cerebral palsy

PURPOSE

This study reports a client-centred development of a non-contact access switch based on an infrared thermal imaging of mouth opening-closing activity of an individual with severe spastic quadriplegic cerebral palsy.

METHOD

Over a 6-month period, the client participated in five test sessions to inform the development of an infrared thermal switch. The client completed eight stimulus-response trials (switch test) and eight word-matching trials (scan test) using the infrared thermal switch and provided subjective feedback throughout.

RESULTS

For the switch test, the client achieved an average correct activation rate of 90% and average response time of 2.4 s. His mean correct activation rate on the scan test improved from 65 to 80% over the course of system development, with an average response time of 11.7 s.

CONCLUSIONS

An infrared thermography switch tuned to a client's extant orofacial gestures is a practical non-invasive access solution and warrants further research in clients with severe physical disability.

A cardiorespiratory classifier of voluntary and involuntary electrodermal activity

BACKGROUND

Electrodermal reactions (EDRs) can be attributed to many origins, including spontaneous fluctuations of electrodermal activity (EDA) and stimuli such as deep inspirations, voluntary mental activity and startling events. In fields that use EDA as a measure of psychophysiological state, the fact that EDRs may be elicited from many different stimuli is often ignored. This study attempts to classify observed EDRs as voluntary (i.e., generated from intentional respiratory or mental activity) or involuntary (i.e., generated from startling events or spontaneous electrodermal fluctuations).

METHODS

Eight able-bodied participants were subjected to conditions that would cause a change in EDA: music imagery, startling noises, and deep inspirations. A user-centered cardiorespiratory classifier consisting of 1) an EDR detector, 2) a respiratory filter and 3) a cardiorespiratory filter was developed to automatically detect a participant's EDRs and to classify the origin of their stimulation as voluntary or involuntary.

RESULTS

Detected EDRs were classified with a positive predictive value of 78%, a negative predictive value of 81% and an overall accuracy of 78%. Without the classifier, EDRs could only be correctly attributed as voluntary or involuntary with an accuracy of 50%.

CONCLUSIONS

The proposed classifier may enable investigators to form more accurate interpretations of electrodermal activity as a measure of an individual's psychophysiological state.

Classification of prefrontal activity due to mental arithmetic and music imagery using hidden Markov models and frequency domain near-infrared spectroscopy

Near-infrared spectroscopy (NIRS) has recently been investigated as a non-invasive brain-computer interface (BCI). In particular, previous research has shown that NIRS signals recorded from the motor cortex during left- and right-hand imagery can be distinguished, providing a basis for a two-choice NIRS-BCI. In this study, we investigated the feasibility of an alternative two-choice NIRS-BCI paradigm based on the classification of prefrontal activity due to two cognitive tasks, specifically mental arithmetic and music imagery. Deploying a dual-wavelength frequency domain near-infrared spectrometer, we interrogated nine sites around the frontopolar locations (International 10-20 System) while ten able-bodied adults performed mental arithmetic and music imagery within a synchronous shape-matching paradigm. With the 18 filtered AC signals, we created task- and subject-specific maximum likelihood classifiers using hidden Markov models. Mental arithmetic and music imagery were classified with an average accuracy of 77.2% +/- 7.0 across participants, with all participants significantly exceeding chance accuracies. The results suggest the potential of a two-choice NIRS-BCI based on cognitive rather than motor tasks.

Classifying affective states using thermal infrared imaging of the human face

In this paper, time, frequency, and time-frequency features derived from thermal infrared data are used to discriminate between self-reported affective states of an individual in response to visual stimuli drawn from the International Affective Pictures System. A total of six binary classification tasks were examined to distinguish baseline and affect states. Affect states were determined from subject-reported levels of arousal and valence. Mean adjusted accuracies of 70% to 80% were achieved for the baseline classifications tasks. Classification accuracies between high and low ratings of arousal and valence were between 50% and 60%, respectively. Our analysis showed that facial thermal infrared imaging data of baseline and other affective states may be separable. The results of this study suggest that classification of facial thermal infrared imaging data coupled with affect models can be used to provide information about an individual's affective state for potential use as a passive communication pathway.

Single-trial classification of NIRS signals during emotional induction tasks: towards a corporeal machine interface

Background

Corporeal machine interfaces (CMIs) are one of a few available options for restoring communication and environmental control to those with severe motor impairments. Cognitive processes detectable solely with functional imaging technologies such as near-infrared spectroscopy (NIRS) can potentially provide interfaces requiring less user training than conventional electroencephalography-based CMIs. We hypothesized that visually-cued emotional induction tasks can elicit forehead hemodynamic activity that can be harnessed for a CMI.

Methods

Data were collected from ten able-bodied participants as they performed trials of positively and negatively-emotional induction tasks. A genetic algorithm was employed to select the optimal signal features, classifier, task valence (positive or negative emotional value of the stimulus), recording site, and signal analysis interval length for each participant. We compared the performance of Linear Discriminant Analysis and Support Vector Machine classifiers. The latency of the NIRS hemodynamic response was estimated as the time required for classification accuracy to stabilize.

Results

Baseline and activation sequences were classified offline with accuracies upwards of 75.0%. Feature selection identified common time-domain discriminatory features across participants. Classification performance varied with the length of the input signal, and optimal signal length was found to be feature-dependent. Statistically significant increases in classification accuracy from baseline rates were observed as early as 2.5 s from initial stimulus presentation.

Conclusion

NIRS signals during affective states were shown to be distinguishable from baseline states with classification accuracies significantly above chance levels. Further research with NIRS for corporeal machine interfaces is warranted.

Infrared thermography as an access pathway for individuals with severe motor impairments

BACKGROUND

People with severe motor impairments often require an alternative access pathway, such as a binary switch, to communicate and to interact with their environment. A wide range of access pathways have been developed from simple mechanical switches to sophisticated physiological ones. In this manuscript we report the inaugural investigation of infrared thermography as a non-invasive and non-contact access pathway by which individuals with disabilities can interact and perhaps eventually communicate.

METHODS

Our method exploits the local temperature changes associated with mouth opening/closing to enable a highly sensitive and specific binary switch. Ten participants (two with severe disabilities) provided examples of mouth opening and closing. Thermographic videos of each participant were recorded with an infrared thermal camera and processed using a computerized algorithm. The algorithm detected a mouth open-close pattern using a combination of adaptive thermal intensity filtering, motion tracking and morphological analysis.

RESULTS

High detection sensitivity and low error rate were achieved for the majority of the participants (mean sensitivity of all participants: 88.5% +/- 11.3; mean specificity of all participants: 99.4% +/- 0.7). The algorithm performance was robust against participant motion and changes in the background scene.

CONCLUSION

Our findings suggest that further research on the infrared thermographic access pathway is warranted. Flexible camera location, convenience of use and robustness to ambient lighting levels, changes in background scene and extraneous body movements make this a potential new access modality that can be used night or day in unconstrained environments.

Infrared thermal imaging as a physiological access pathway: a study of the baseline characteristics of facial skin temperatures

In this study we examine the baseline characteristics of facial skin temperature, as measured by dynamic infrared thermal imaging, to gauge its potential as a physiological access pathway for non-verbal individuals with severe motor impairments. Frontal facial recordings were obtained from 12 asymptomatic adults in a resting state with a high-end infrared thermal imaging system. From the infrared thermal recordings, mean skin temperature time series were generated for regions of interest encompassing the nasal, periorbital and supraorbital areas. A 90% bandwidth for all regions of interest was found to be in the 1 Hz range. Over 70% of the time series were identified as nonstationary (p<0.05), with the nonstationary mean as the greatest contributing source. Correlation coefficients between regions were significant (p<0.05) and ranged from values of 0.30 (between periorbital and supraorbital regions) to 0.75 (between contralateral supraorbital regions). Using information measures, we concluded that the greatest degree of information existed in the nasal and periorbital regions. Mutual information existed across all regions but was especially prominent between the nasal and periorbital regions. Results from this study provide insight into appropriate analysis methods and potential discriminating features for the application of facial skin temperature as a physiological access pathway.