||This article may present fringe theories, without giving appropriate weight to the mainstream view, and explaining the responses to the fringe theories. (March 2016)|
|Some of this article's listed sources may not be reliable. (March 2016) (Learn how and when to remove this template message)|
||This article possibly contains original research. (June 2016) (Learn how and when to remove this template message)|
||This article may be too technical for most readers to understand. (March 2016) (Learn how and when to remove this template message)|
Brainwave entrainment is a colloquialism for 'neural entrainment', which denotes how the aggregate oscillation frequency, resulting from synchronous electrical activity among ensembles of cortical neurons, can adjust to synchronize with the periodic vibration of an external stimulus, such as a sustained acoustic frequency perceived as pitch, a regularly repeating pattern of intermittent sounds perceived as rhythm, or a regularly intermittent flashing light.
- 1 Neural oscillation and electroencephalography (EEG)
- 2 Frequency bands of cortical neural ensembles
- 3 Neural oscillation and cognitive functions
- 4 Awareness and consciousness
- 5 Entrainment
- 6 Music and the frequency following response
- 7 Binaural beats
- 8 See also
- 9 References
- 10 Further reading
- 11 External links
Neural oscillation and electroencephalography (EEG)
Neural oscillations are rhythmic or repetitive electrochemical activity in the brain and central nervous system. Such oscillations can be characterized by their frequency, amplitude and phase. Neural tissue can generate oscillatory activity driven by mechanisms within individual neurons, as well as by interactions between them. They may also adjust frequency to synchronize with the periodic vibration of external acoustic or visual stimuli.
The activity of neurons generate electric currents; and the synchronous action of neural ensembles in the cerebral cortex, comprising large numbers of neurons, produce macroscopic oscillations, which can be monitored and graphically documented by an electroencephalogram (EEG). The electroencephalographic representations of those oscillations are typically denoted by the term 'brainwaves' in common parlance.
The technique of recording neural electrical activity within the brain from electrochemical readings taken from the scalp originated with the experiments of Richard Caton in 1875, whose findings were developed into electroencephalography (EEG) by Hans Berger in the late 1920s.
Frequency bands of cortical neural ensembles
The fluctuating frequency of oscillations generated by the synchronous activity of cortical neurons, measurable with an electroencephalogram (EEG), via electrodes attached to the scalp, are conveniently categorized into general bands, in order of decreasing frequency, measured in Hertz (HZ) as follows:
Neural oscillation and cognitive functions
The functional role of neural oscillations is still not fully understood; however they have been shown to correlate with emotional responses, motor control, and a number of cognitive functions including information transfer, perception, and memory. Specifically, neural oscillations, in particular theta activity, are extensively linked to memory function, and coupling between theta and gamma activity is considered to be vital for memory functions, including episodic memory.
Awareness and consciousness
Electroencephalography (EEG) has been most widely used in the study of neural activity generated by large groups of neurons, known as neural ensembles, including investigations of the changes that occur in electroencephalographic profiles during cycles of sleep and wakefulness. EEG signals change dramatically during sleep and show a transition from faster frequencies to increasingly slower frequencies, indicating a relationship between the frequency of neural oscillations and cognitive states including awareness and consciousness.
Meaning and origin of the term 'entrainment'
Entrainment is a term originally derived from complex systems theory, and denotes the way that two or more independent, autonomous oscillators with differing rhythms or frequencies, when situated in a context and at a proximity where they can interact for long enough, influence each other mutually, to a degree dependent on coupling force, such that they adjust until both oscillate with the same frequency. Examples include the mechanical entrainment or cyclic synchronization of two electric clothes dryers placed in close proximity, and the biological entrainment evident in the synchronized illumination of fireflies.
Entrainment is a concept first identified by the Dutch physicist Christiaan Huygens in 1665 who discovered the phenomenon during an experiment with pendulum clocks: He set them each in motion and found that when he returned the next day, the sway of their pendulums had all synchronized.
Such entrainment occurs because small amounts of energy are transferred between the two systems when they are out of phase in such a way as to produce negative feedback. As they assume a more stable phase relationship, the amount of energy gradually reduces to zero, with system of greater frequency slowing down, and the other speeding up.
Subsequently, the term 'entrainment' has been used to describe a shared tendency of many physical and biological systems to synchronize their periodicity and rhythm through interaction. This tendency has been identified as specifically pertinent to the study of sound and music generally, and acoustic rhythms specifically. The most ubiquitous and familiar examples of neuromotor entrainment to acoustic stimuli is observable in spontaneous foot or finger tapping to the rhythmic beat of a song.
Exogenous rhythmic entrainment, which occurs outside the body, has been identified and documented for a variety of human activities, which include the way people adjust the rhythm of their speech patterns to those of the subject with whom they communicate, and the rhythmic unison of an audience clapping.
Even among groups of strangers, the rate of breathing, locomotive and subtle expressive motor movements, and rhythmic speech patterns have been observed to synchronize and entrain, in response to an auditory stimuli, such as a piece of music with a consistent rhythm. Furthermore, motor synchronization to repetitive tactile stimuli occurs in animals, including cats and monkeys as well as humans, with accompanying shifts in electroencephalogram (EEG) readings.
Examples of endogenous entrainment, which occurs within the body, include the synchronizing of human circadian sleep-wake cycles to the 24-hour cycle of light and dark. and the synchronization of a heartbeat to a cardiac pacemaker.
Brainwaves, or neural oscillations, share the fundamental constituents with acoustic and optical wave forms, including frequency, amplitude, and periodicity. Consequently, Huygens' discovery precipitated inquiry into whether or not the synchronous electrical activity of cortical neural ensembles might not only alter in response to external acoustic or optical stimuli but also entrain or synchronize their frequency to that of a specific stimulus.
Brainwave entrainment is a colloquialism for such 'neural entrainment', which is a term used to denote the way in which the aggregate frequency of oscillations produced by the synchronous electrical activity in ensembles of cortical neurons can adjust to synchronize with the periodic vibration of an external stimuli, such as a sustained acoustic frequency perceived as pitch, a regularly repeating pattern of intermittent sounds, perceived as rhythm, or of a regularly rhythmically intermittent flashing light.
Music and the frequency following response
Changes in neural oscillations, demonstrable through electroencephalogram (EEG) measurements, are precipitated by listening to music, which can modulate autonomic arousal ergotropically and trophotropically, increasing and decreasing arousal respectively. Musical auditory stimulation has also been demonstrated to improve immune function, facilitate relaxation, improve mood, and contribute to the alleviation of stress. These findings have contributed to the development of neurologic music therapy, which uses music and song as an active and receptive intervention, to contribute to the treatment and management of disorders characterized by impairment to parts of the brain and central nervous system, including stroke, traumatic brain injury, Parkinson's disease, Huntington's disease, cerebral palsy, Alzheimer's disease, and autism.
Meanwhile, the therapeutic benefits of listening to sound and music is a well-established principle upon which the practice of receptive music therapy is founded. The term 'receptive music therapy' denotes a process by which patients or participants listen to music with specific intent to therapeutically benefit; and is a term used by therapists to distinguish it from 'active music therapy' by which patients or participants engage in producing vocal or instrumental music. Receptive music therapy is an effective adjunctive intervention suitable for treating a range of physical and mental conditions.
The Frequency following response (FFR), also referred to as Frequency Following Potential (FFP), is a specific response to hearing sound and music, by which neural oscillations adjust their frequency to match the rhythm of auditory stimuli. The use of sound with intent to influence cortical brainwave frequency is called auditory driving, by which frequency of neural oscillation is 'driven' to entrain with that of the rhythm of a sound source.
Human subjects rarely hear frequencies below 20 Hz, which is exactly the range of Delta, Theta, Alpha, and low to mid Beta brainwaves. Among the methods by which some investigations have sought to overcome this problem is to measure electroencephalogram (EEG) readings of a subject while he or she listens to binaural beats.
A binaural beat is an auditory illusion perceived when two different pure-tone sine waves, both with frequencies lower than 1500 Hz, with less than a 40 Hz difference between them, are presented to a listener dichotically, that is one through each ear. For example, if a 530 Hz pure tone is presented to a subject's right ear, while a 520 Hz pure tone is presented to the subject's left ear, the listener will perceive the auditory illusion of a third tone, in addition to the two pure-tones presented to each ear. The third sound is called a binaural beat, and in this example would have a perceived pitch correlating to a frequency of 10 Hz, that being the difference between the 530 Hz and 520 Hz pure tones presented to each ear.
Binaural-beat perception originates in the inferior colliculus of the midbrain and the superior olivary complex of the brainstem, where auditory signals from each ear are integrated and precipitate electrical impulses along neural pathways through the reticular formation up the midbrain to the thalamus, auditory cortex, and other cortical regions  Listening to binaural beats has been shown to precipitate auditory driving by which ensembles of cortical neurons entrain their frequencies to that of the binaural beat, with associated changes in self-reported subjective experience of emotional and cognitive state. It is however possible that the real cause of the observed changes is the headphones, rather than the sound itself, as demonstrated in 2002 during a University of Virginia presentation at the Society for Psychophysiologial Research. It was shown that EEG changes did not occur when the standard electromagnetic headphones were replaced by air conduction headphones, which were connected to a remote transducer by rubber tubes. This suggests that the basis for the entrainment effects is electromagnetic rather than acoustical.
- Niedermeyer E. and da Silva F.L., Electroencephalography: Basic Principles, Clinical Applications, and Related Fields. Lippincot Williams & Wilkins, 2004.
- da Silva, F. L., Neural mechanisms underlying brain waves: from neural membranes to networks. Electroencephalography and Clinical Neurophysiology, Vol. 79, No. 2, 1991, pp81-93.
- Cooper, R., Winter, A., Crow, H., and Walter, W. G., Comparison of subcortical, cortical, and scalp activity using chronically indwelling electrodes in man. Electroencephalography and Clinical Neurophysiology, Vol. 18, 1965, pp217–230.
- da Silva, F. H., and van Leeuwan, W., The cortical alpha rhythm in and the depth and surface profile of phase. In Brazier, M. A. B. and Petsche, H., (Eds.), Architectonics of the Cerebral Cortex. New York, NY: Raven Press, 1978.
- da Silva, F. H., Neural mechanism underlying brain waves: From neural membranes to networks. Electroencephalography and Clinical Neurophysiology, Vol. 79, 1991, pp81–93.
- Deuschl, G., and Eisen, A., Recommendations for the practice of clinical neurophysiology. Guidelines of the International Federation of Clinical Neurophysiology. Electroencephalography and Clinical Neurophysiology Supplement, 1999.
- Llinas, R. R. (2014). "Intrinsic electrical properties of mammalian neurons and CNS function: a historical perspective". Front Cell Neurosci 8: 320. doi:10.3389/fncel.2014.00320. PMC 4219458. PMID 25408634.
- Fries P (2005). "A mechanism for cognitive dynamics: neuronal communication through neuronal coherence". TICS 9 (10): 474–480. doi:10.1016/j.tics.2005.08.011.
- Fell J, Axmacher N (2011). "The role of phase synchronization in memory processes". Nature Reviews Neuroscience 12 (2): 105–118. doi:10.1038/nrn2979. PMID 21248789.
- Schnitzler A, Gross J (2005). "Normal and pathological oscillatory communication in the brain". Nature Reviews Neuroscience 6 (4): 285–296. doi:10.1038/nrn1650. PMID 15803160.
- Buszaki G (2006). Rhythms of the brain. Oxford University Press.
- Nyhus, E; Curran T (June 2010). "Functional role of gamma and theta oscillations in episodic memory". Neuroscience and Biobehavioral Reviews 34 (7): 1023–1035. doi:10.1016/j.neubiorev.2009.12.014. PMC 2856712. PMID 20060015.
- Rutishauser U, Ross IB, Mamelak AN, Schuman EM (2010). "Human memory strength is predicted by theta-frequency phase-locking of single neurons". Nature 464 (7290): 903–907. doi:10.1038/nature08860. PMID 20336071.
- Engel AK, Singer W (2001). "Temporal binding and the neural correlates of sensory awareness". Trends in Cognitive Sciences 5 (1): 16–25. doi:10.1016/S1364-6613(00)01568-0. PMID 11164732.
- Varela F, Lachaux JP, Rodriguez E, Martinerie J (2001). "The brainweb: phase synchronization and large-scale integration". Nature Reviews Neuroscience 2 (4): 229–239. doi:10.1038/35067550. PMID 11283746.
- Néda, Z., Ravasz, E., Brechet, Y., Vicsek, T., & Barabsi, A. L., Self-organizing process: The sound of many hands clapping. Nature, Vol. 403, 2000, pp849–850.
- Pantaleone, J., Synchronization of Metronomes. American Journal of Physics, Vol. 70, 2002 pp992–1000.
- Bennett, M., Schatz, M. F., Rockwood, H., and Wiesenfeld, K., Huygens's clocks. Proceedings: Mathematics, Physical and Engineering Sciences, 2002, pp563-579.
- Haas, F., Distenfeld, S., & Axen, K., Effects of perceived musical rhythm on respiratory pattern. Journal of Applied Physiology, Vol. 61, No. 3, 1986, pp1185–1191.
- Safranek, M., Koshland, G., and Raymond, G., Effect of auditory rhythm on muscle activity. Physical Therapy, Vol. 62, 1982, pp161–168.
- Thaut, M.H., Schleiffers, S., and Davis, W.B., Changes in EMG patterns under the influence of auditory rhythm. In Spintge, R. and Droh, R. (Eds.), Music Medicine St. Louis, MO: MMB Music, 1992.
- Thaut, M. H., McIntosh, G. C., Prassas, S. G., and Rice, R. R., Effect of rhythmic cuing on temporal stride parameters and EMG patterns in hemiparetic stroke patients. Journal of Neurologic Rehabilitation, Vol. 7, 1993, pp9–16.
- Thaut, M., McIntosh, G., Prassas, S., and Rice, R., Effect of rhythmic cuing on temporal stride parameters and EMG patterns in normal gait. Journal of Neurologic Rehabilitation, Vol. 6, 1992, pp185–190.
- McIntosh, G.C., Thaut, M.H., and Rice, R.R., 1996. Rhythmic auditory stimulation as entrainment and therapy technique in gait of stroke and Parkinson’s disease patients. In Pratt, R. and. Spintge, R., (Eds.), Music Medicine. St. Louis, MO: MMB Music, 1996.
- Condon, W. S., Multiple response to sound in dysfunctional children. Journal of Autism and Childhood Schizophrenia, Vol. 5, No. 1, 1975, p43.
- Pompeiano, O., and Swett, J. E., EEG and behavioral manifestations of sleep induced by cutaneous nerve stimulation in normal cats. Archives Italiennes de Biologie, Vol. 100, 1962, pp311–342.
- Walter, D. O., and Adey, W. R., Linear and nonlinear mechanisms of brainwave generation. Annals of the New York Academy of Sciences, Vol. 128, 1966, pp772–780.
- Namerow, N. S., Sclabassi, R. J., and Enns, N. F., Somatosensory responses to stimulus trains: Normative data. Electroencephalography and Clinical Neurophysiology, Vol. 37, 1974, pp11–21.
- Gavalas, R. J., Walter, D. O., Hamer, J., and Adey, W. R., Effects of low-level, low-frequency electric fields on EEG and behavior in Macaca uemestriua. Brain Research, Vol. 18, 1970, pp491–501.
- Buzsáki, G., Rhythms of the Brain. New York, NY: Oxford University Press, 2006.
- Clayton M., Sager R., and Will U., In time with the music: the concept of entrainment and its significance for ethnomusicology. In European Meetings in Ethnomusicology Vol. 11, 2005, pp3-142.
- Cvetkovic D., Powers R., and Cosic I., Preliminary evaluation of electroencephalographic entrainment using thalamocortical modelling. Expert Systems, Vol. 26, 2009, pp320-338.
- Will, U., and Berg, E., Brainwave synchronization and entrainment to periodic stimuli. Neuroscience Letters, Vol. 424, 2007, pp55–60.
- Cade, G. M. and Coxhead, F., The awakened mind, biofeedback and the development of higher states of awareness. New York, NY: Delacorte Press, 1979.
- Neher, A., Auditory driving observed with scalp electrodes in normal subjects. Electroencephalography and Clinical Neurophysiology, Vol. 13, 1961, pp449–451.
- Zakharova, N. N., and Avdeev, V. M., Functional changes in the central nervous system during music perception. Zhurnal vysshei nervnoi deiatelnosti imeni IP Pavlova Vol. 32, No. 5, 1981, pp915-924.
- Wagner, M. J., Brainwaves and biofeedback. A brief history - Implications for music research. Journal of Music Therapy, Vol. 12, No. 2, 1975, pp46-58.
- Fikejz, F., Influence of music on human electroencephalogram. In Applied Electronics (AE), International Conference, 2011.
- Ogata, S., Human EEG responses to classical music and simulated white noise: effects of a musical loudness component on consciousness. Perceptual and Motor Skills Vol. 80, No. 3, 1995, pp779-790.
- Lin, Y. P., Yang, Y. H., and Jung, T. P., Fusion of electroencephalographic dynamics and musical contents for estimating emotional responses in music listening. Frontiers in Neuroscience, Vol. 8, 2014.
- Nakamura, S., Sadato, N., Oohashi, T., Nishina, E., Fuwamoto, Y., and Yonekura, Y., Analysis of music–brain interaction with simultaneous measurement of regional cerebral blood flow and electroencephalogram beta rhythm in human subjects. Neuroscience letters, Vol. 275, No. 3, 1999, pp222-226.
- Karthick, N. G., Thajudin, A. V. I., and Joseph, P. K., Music and the EEG: a study using nonlinear methods. In Biomedical and Pharmaceutical Engineering, 2006. Biomedical and Pharmaceutical Engineering, International Conference, Singapore, 2006.
- Trost W. and Vuilleumier P., Rhythmic entrainment as a mechanism for emotion induction by music: a neurophysiological perspective. In The Emotional Power of Music: Multidisciplinary Perspectives on Musical Arousal, Expression, and Social Control, Cochrane T., Fantini B., and Scherer K. R., (Eds.), Oxford, UK: Oxford University Press; 2013, pp213–225.
- Szabó, C., The effects of monotonous drumming on subjective experiences. Music Therapy Today, Vol. 1, 2004, 2004, pp. 1-9.
- Bittman, B. B., Berk, L. S., Felten, D. L., Westengard, J., Simonton, O. C., Pappas, J., and Ninehouser, M., Composite effects of group drumming music therapy on modulation of neuroendocrine-immune parameters in normal subjects. Alternative Therapeutic Health Medicine, Vol. 1, 2001, pp38–47.
- Wachiuli, M., Koyama, M., Utsuyama, M., Bittman, B. B., Kitagawa, M., and Hirokawa, K., Recreational music-making modulates natural killer cell activity, cytokines, and mood states in corporate employees. Medical Science Monitor, Vol. 13, No. 2, 2007, CR57–70.
- Bittman, B., Bruhn, K. T., Stevens, C., & Westengard, J., and Umbach, P. O., Recreational music-making: A cost-effective group interdisciplinary strategy for reducing burnout and improving mood states in long-term care workers. Advanced Mind Body Medicine, Vol. 19, Nos. 3-4, 2003, p16.
- Bittman, B. B., Snyder, C., Bruhn, K. T., Liebfreid, F., Stevens, C. K., Westengard, J., and Umbach, P. O., Recreational music-making: An integrative group intervention for reducing burnout and improving mood states in first year associate degree nursing students: Insights and economic impact. International Journal of Nursing Education Scholarship, Vol. 1, Article 12, 2004.
- Walton, K., and Levitsky, D., A neuroendocrine mechanism for the reduction of drug use and addictions by transcendental meditation. In O’Connell, D. and Alexander, C. (Eds.), Self-recovery: Treating addictions using transcendental meditation and Maharishi Ayur-Veda. New York, NY: Haworth, 1994.
- Szabó, C., The effects of monotonous drumming on subjective experiences. Music Therapy Today, Vol. 1, 2004, pp. 1–9.
- Winkelman, M., Complementary therapy for addiction: Drumming out drugs. The American Journal of Public Health, Vol. 93, 2003, pp647–651.
- Thaut, M. H., Peterson, D. A., & McIntosh, G. C. (2005). Temporal entrainment of cognitive functions. Annals of the New York Academy of Sciences, 1060(1), 243-254
- Thaut, M.,Training manual for neurologic music therapy. Colorado State University: Center for Biomedical Research in Music, 1999.
- Thaut, M. H., Neurologic music therapy in cognitive rehabilitation. Music Perception, Vol. 27, No. 4, 2010, pp281-285.
- Bruscia, K., Defining music therapy. Barcelona: Gilsum, NH, 1998.
- Grocke, D., and Wigram, T. (2007). Receptive methods in music therapy: Techniques and clinical applications for music therapy clinicians, educators, and students. London, England: Jessica Kingsley, 2007.
- Burkard, R., Don, M., and Eggermont, J. J., Auditory evoked potentials: Basic principles and clinical application. Philadelphia, PA: Lippincott Williams & Wilkins, 2007.
- Worden, F.G.; Marsh, J.T., Frequency-following (microphonic-like) neural responses evoked by sound. Electroencephalography and Clinical Neurophysiology Vol. 25, No. 1, 1968, pp42–52. doi:10.1016/0013-4694(68)90085-0.
- Neher, Andrew. Auditory driving observed with scalp electrodes in normal subjects. Electroencephalography and Clinical Neurophysiology 13.3 (1961): 449-451. doi:10.1016/0013-4694(61)90014-1.
- Wright, Peggy A. "Rhythmic drumming in contemporary shamanism and its relationship to auditory driving and risk of seizure precipitation in epileptics." Anthropology of Consciousness 2.3‐4 (1991): 7-14. doi:10.1525/ac.1991.2.3-4.7.
- Rosen, S. and Howell, P., Signals and Systems for Speech and Hearing. Bingley, UK: Emerald, 2001.
- Rossing, T., (2007). Springer Handbook of Acoustics. Berlin, Springer: 2007.
- McConnell, P. A., Froeliger, B., Garland, E. L., Ives, J. C., & Sforzo, G. A., Auditory driving of the autonomic nervous system: Listening to theta-frequency binaural beats post-exercise increases parasympathetic activation and sympathetic withdrawal. Frontiers in Psychology, Vol. 5, p2014.
- Draganova R., Ross B., Wollbrink A., Pantev C. (2008). Cortical steady-state responses to central and peripheral auditory beats. Cerebral Cortex Vol. 18, 2008, pp1193–1200.
- Smith J. C., Marsh J. T. and Brown W. S. Far-field recorded frequency-following responses: evidence for the locus of brainstem sources. Electroencephalogr. Clin. Neurophysiol. Vol., 1975, pp465–472.
- Oster, G., Auditory beats in the brain. Scientific American, Vol. 229, No. 4, 1973, pp94-102.
- Swann R., Bosanko S., Cohen R., Midgley R., Seed K. M.,The Brain - A User’s Manual. New York, NY: G. P. Putnam and Sons, 1982.
- Draganova R., Ross B., Wollbrink A., Pantev C., Cortical steady-state responses to central and peripheral auditory beats. Cerebral Cortex Vol. 18, 2008, pp1193-1200.
- Becher, A. K., Höhne, M., Axmacher, N., Chaieb, L., Elger, C. E., and Fell, J., Intracranial electroencephalography power and phase synchronization changes during monaural and binaural beat stimulation. European Journal of Neuroscience, Vol. 41, No. 2, 2015, pp. 254-263. doi:10.1111/ejn.12760. PMID 25345689.
- Pratt, H., Starr, A., Michalewski, H. J., Dimitrijevic, A., Bleich, N., and Mittelman, N., Cortical evoked potentials to an auditory illusion: binaural beats. Clinical neurophysiology, Vol. 120, No. 8, 2009, pp1514-1524.
- Karino, S., Yumoto, M., Itoh, K., Uno, A., Yamakawa, K., Sekimoto, S., and Kaga, K. (2006). Neuromagnetic responses to binaural beat in human cerebral cortex. Journal of neurophysiology, Vol. 96, No. 4, 2006, pp1927-1938.
- McConnell, P. A., Froeliger, B., Garland, E. L., Ives, J. C., & Sforzo, G. A., Auditory driving of the autonomic nervous system: Listening to theta-frequency binaural beats post-exercise increases parasympathetic activation and sympathetic withdrawal. Frontiers in Psychology, Vol. 5, 2014.
- Chandra Stone, Phyllis Thomas, Dennis McClain-Furmanski, & James E. Horton (2002). "EEG oscillations and binaural beat as compared with electromagnetic headphones and air-conduction headphones", Psychophysiology vol 39, pp. S80
- Thaut, M. H., Rhythm, Music, and the Brain: Scientific Foundations and Clinical Applications (Studies on New Music Research). New York, NY: Routledge, 2005.
- Berger, J. and Turow, G. (Eds.), Music, Science, and the Rhythmic Brain : Cultural and Clinical Implications. New York, NY: Routledge, 2011.